Semiconductor laser

Abstract

A semiconductor laser comprises: a first cladding layer of a first conduction type; an active layer stacked on the first cladding layer; and a second cladding layer of a second conduction type stacked on the active layer. The first cladding layer, the active layer and the second cladding layer are made of II-VI compound semiconductors. Pulse oscillation occurs with characteristics of a threshold current i_th (A), a threshold voltage v_th (v) of the diode composed of the first cladding layer, the active layer and the second cladding layer, a differential resistance r_S (Q) of the diode after the rising, a thermal resistance r_t (K/W) and a characteristic temperature t₀ (K). When two amounts α and β are defined by:

α.tbd.(r_t /t₀)i_th v_th

β.tbd.(r_t /t₀)r_S t_th²

the point (α,β) exists in an area on the α-β plane surrounded by the straight line α=0, the straight line β=0, and the curve ((2ln t-1)/t, (1-ln t)/t²) having t as a parameter. The semiconductor laser may include a first optical waveguide layer between the first cladding layer and the active layer and include a second optical waveguide layer between the second cladding layer and the active layer, the first optical waveguide layer and the second optical waveguide layer being made of II-VI compound semiconductors. II-VI compound semiconductors making the first cladding layer and the second cladding layer may be a ZnMgSSe compound semiconductor. A semiconductor laser using II-VI compound semiconductors and having the capability of continuous oscillation at high temperatures including the room temperature is provided.

Description

BACKGROUND OF THE INVENTION

This invention relates to a semiconductor laser capable of emitting, for example, blue to green light.

In recent years, there is a demand for semiconductor lasers capable of emitting light with short wavelengths for improving recording densities of optical disks or resolutions of laser printers, and vigorous studies have been made for realization of such lasers.

II-VI compound semiconductors are hopeful as materials to be used for fabricating such semiconductor lasers capable of emitting light with short wavelengths. In particular, ZnMgSSe compound semiconductors, which are quaternary II-VI compound semiconductors, are recognized to be suitable as materials of cladding layers and optical waveguide layers required to make on a GaAs substrate a semiconductor laser for emitting blue to green light in the band of wavelengths of 400 nm ∼500 nm (see, for example, Electron. Lett. 28 (1992) 1798).

As to semiconductor lasers using II-VI compound semiconductors, active studies have been made to realize oscillation at the room temperature. As a result, the present Applicant has succeeded in pulse oscillation at the room temperature by a semiconductor laser using ZnMgSSe compound semiconductors referred to above.

As to continuous oscillation at the room temperature, however, there has been no report either with a semiconductor laser using ZnMgSSe compound semiconductors mentioned above or with a semiconductor laser using II-VI compound semiconductors other than ZnMgSSe compound semiconductors.

OBJECTS AND SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a semiconductor laser using a II-VI compound semiconductor as the material of cladding layers and having the capability of continuous oscillation at high temperatures including the room temperature.

According to an aspect of the invention, there is provided a semiconductor laser comprising: a first cladding layer of a first conduction type; an active layer stacked on the first cladding layer; and a second cladding layer of a second conduction type stacked on the active layer, wherein:

the first cladding layer, the active layer and the second cladding layer are made of II-VI compound semiconductors, and

pulse oscillation occurs with characteristics of a threshold current I_th (A), a threshold voltage v_th (v) of the diode composed of the first cladding layer, the active layer and the second cladding layer, a differential resistance R_S (Q) of the diode after the rising, a thermal resistance R_t (K/W) and a characteristic temperature T₀ (K), and wherein:

when two amounts α and β are defined by:

α.tbd.(R_t /T₀)I_th v_th

β.tbd.(R_t /T₀)R_S I_th²

the point (α,β) exists in an area on the α-β plane surrounded by the straight line α=0, the straight line β=0, and the curve ((2ln t-1)/t, (1-ln t)/t²) having t as a parameter.

According to another aspect of the invention, the semiconductor laser includes a first optical waveguide layer between the first cladding layer and the active layer and includes a second optical waveguide layer between the second cladding layer and the active layer, the first optical waveguide layer and the second optical waveguide layer being made of II-IV compound semiconductors.

According to still another aspect of the invention, II-VI compound semiconductors making the first cladding layer and the second cladding layer are a ZnMgSSe compound semiconductor.

The above, and other, objects, features and advantage of the present invention will become readily apparent from the following detailed description thereof which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph for explaining the principle of the invention;

FIG. 2 is a graph for explaining the principle of the invention;

FIG. 3 is a cross-sectional view showing a semiconductor laser according to a first embodiment of the invention;

FIG. 4 is a cross sectional view of the semiconductor laser according to the first embodiment of the invention;

FIG. 5 is a cross-sectional view showing the semiconductor laser according to the first embodiment of the invention mounted on a heat sink;

FIG. 6 is a schematic view showing a MBE apparatus used for fabricating the semiconductor laser according to the first embodiment of the invention;

FIG. 7 is a graph showing a result of measurement of characteristics of the semiconductor laser according to first embodiment of the invention in term of the light output relative to the current at the room temperature;

FIG. 8 is a graph showing a result of measurement of the spectrum of emitted light at the room temperature by the semiconductor laser according to the first embodiment of the invention;

FIG. 9 is a cross-sectional view showing a semiconductor laser according to a second embodiment of the invention;

FIG. 10 is an energy band diagram showing valence bands near the boundary between p-type ZnSe and p-type ZnTe;

FIG. 11 is a graph showing changes in first quantum level E₁ with the width L_W of a p-type ZnTe quantum well; and

FIG. 12 is an energy band diagram showing a design of a p-type of ZnTe/ZnSe MQW layer in the semiconductor laser according to the second embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Assume that a semiconductor laser is driven with a duty k, the corresponding threshold current is I_th (A), and the corresponding operating voltage is V_op (V). I_th and V_op become functions of k, and k=1 corresponds to continuous driving of the semiconductor laser.

In this case, if the semiconductor laser is operated just above the threshold value for oscillation, the time average P(W) of power supplied to the semiconductor laser becomes:

p=kI_th v_op (1)

Part of P is emitted as light to the exterior of the semiconductor laser, and the remainder appears as heat of the semiconductor laser.

Since an increase in temperature AT(K) of the semiconductor layer caused by the generation of the heat is considered to be proportional with P,

ΔT=R_t P=kR_t I_th V_op (2)

where R_t (K/W) is an (apparent) thermal resistance of the semiconductor. In this case, if the atmospheric temperature is T_atom (K), then the temperature T(K) of the semiconductor laser is:

T=T_atom +ΔT (3)

On the other hand, when the characteristic temperature is T_O (K), the temperature T and the threshold current I_th of the semiconductor laser have the relation:

I_th (T=T₂)=I_th (T=T₁) exp ((T₂ -T₁)/T_O) (4)

Therefore, when the temperatures of the semiconductor laser operated with duties k₁ and k₂ are T₁ and T₂, respectively, and corresponding threshold currents are I_th (T=T₁)=I_th (k₁) and I_th (T=T₂)=I_th (k₂), then equations (2), (3) and (4) result in: ##EQU1##

Assuming that the current-voltage (I-V) characteristics of the semiconductor laser do not depend on the temperature and are expressed in the form of the function:

V=V_th +R_S I (6)

then, V_op =V_th +R_S I_th. Entering this equation into equation (5) results in:

I_th (k₂)=I_th (k₁) exp ((R_t /T₀){k₂ I_th (k₂)[V_th +R_S I_th (k₂)]-k₁ I_th (k₁)[V_th +R_S I_th (k₁)]}) (7)

In equation (7), if k₁ <<k₂ =1,I_th (pulse).tbd.I_th (k₁), I_th (cw).tbd.I_th (1), it becomes:

I_th (cw)≈I_th (pulse) exp (R_t /T₀){I_th (cw)[V_th +R_S I_th (cw)]}) (8)

By using the following three dimensionless quantities:

x.tbd.I_th (cw)/I_th (pulse) (9)

α=(R_t /T₀)I_th (pulse)V_th (10)

β=(R_t /T₀)R_S I_th² (pulse) (11)

equation (8) can be rewritten as:

x=exp(αx+βx²) (12)

Considering the physical meaning of equation (12), it is known that, when the parameter α indicative of an increase in threshold current due to the threshold voltage V_th and the parameter β indicative of an increase in threshold current due to the serial resistance component R_S are given, if x satisfying equation (12) exists, continuous oscillation is possible with a current x times the threshold current for pulse oscillation in a range in which generation of heat can be disregarded.

By taking logarithms of both sides of equation (12) and by dividing them by x,

ln x/x=α+βx (13)

is obtained. Therefore, the foregoing conditions can be reworded such that, if there exists a point of intersection of the curve:

y=ln x/x (14)

and the straight line:

y=α+βx (15)

then continuous oscillation at the room temperature is possible with the (minimum) value of the x-coordinates.

FIG. 1 shows a graph of y=ln x/x. Since α>0 and β>0, it is directly known that:

1. Unless α21 e- and β<e-1 /2, no point of intersection exists. That is, continuous oscillation is not realized.

2. If a point of intersection exists, then the minimum x is smaller than e. That is, if continuous oscillation is realized, then the threshold current becomes less than e times the value upon pulse oscillation.

It should be noted that 1 is a necessary condition and not a sufficient condition.

β<e-1 /2 is obtained as follows. When α=0, conditions for y=ln x/x to intersect with y=β_MAX x are given by simultaneous equations:

y(x₀)=ln x₀ /x₀ =β_MAX x₀ (16a)

(dy/dx) (x₀)=(1-ln x₀)/x₀² =β_MAX (16b)

where x₀ is the x-coordinate of the point of intersection. To solve the simultaneous equations, β_MAX is first canceled to obtain x₀ =e^1/2 which in turn is substituted in one of simultaneous equations to determine Δ_MAX =e-1 /2. It is apparent from FIG. 1 that β<β_MAX is a necessary condition for existence of a point of intersection.

Necessary and sufficient conditions of a and β to make the curve of equation (14) and the straight line of equation (15) to have a point of intersection will now be obtained. Consideration is made by fixing α at a value in the range of (0, e-1). When the slope of a tangent y=ln x/x with the y-intercept being α is indicated by β_M (α), it is known from FIG. 1 that

β<β_M)α) (17)

is the condition wanted. When the coordinates of the point of contact of the tangent is (x₀, ln x₀ /x₀), the equation of the tangent becomes: ##EQU2##

Therefore, taking x₀ as a parameter, β_M was obtained from:

β=(2ln x₀ -1)/x₀ (19a)

β_M =(1-ln x₀)/x₀² (19b)

A result of actual calculation thereof by using a computer is shown in FIG. 2.

It is known from the foregoing description that, in order that the curve of equation (14) and the straight line of equation (15) have a point of intersection, the point (α,β) must exist in an area D surrounded by the α-axis, i.e. the straight line β=0, the β-axis, i.e. the straight line α=0, and a curve ((2ln x₀ -1)/x₀, (1-ln x0)/x0²) having x₀ as a parameter.

This invention has been made on the basis of the above-realization by the present Inventors.

That is, in order to accomplish the aboveindicated object, there is provided a semiconductor laser comprising: a first cladding layer of a first conduction type; an active layer stacked on the first cladding layer; and a second cladding layer of a second conduction type stacked on the active layer, wherein:

the first cladding layer, the active layer and the second cladding layer are made of II-VI compound semiconductors, and

pulse oscillation occurs with characteristics of a threshold current I_th (A), a threshold voltage V_th (V) of the diode composed of the first cladding layer, the active layer and the second cladding layer (7), a differential resistance R_S (Ω) of the diode after the rising, a thermal resistance R_t (K/W) and a characteristic temperature T₀ (K), and wherein:

when two amounts α and β are defined by:

α.tbd.=(R_t /T₀)I_th V_th

β.tbd.(R_t /T₀)R_S I_th²

the point (α,β) exists in an area on the α-β plane surrounded by the straight line α=0, the straight line β=0, and the curve ((2ln t-1)/t, (1-ln t/t²) having t as a parameter.

In a working form of the semiconductor laser according to the invention, the semiconductor laser includes a first optical waveguide layer between the first cladding layer and the active layer and includes a second optical waveguide layer between the second cladding layer and the active layer, the first optical waveguide layer and the second optical waveguide layer being made of II-IV compound semiconductors.

In a preferred working form of the semiconductor laser according to the invention, II-VI compound semiconductors making the first cladding layer and the second cladding layer are a ZnMgSSe compound semiconductor.

According to the invention, in a semiconductor laser for pulse oscillation with characteristics of a threshold current I_th (A), a threshold voltage V_th (v) of the diode, the differential resistance R_s (Ω) of the diode after the rising, the thermal resistance R_th (K/W), and the characteristic temperature T_O (K), it is possible to realize a semiconductor laser using II-VI compound semiconductors and having the capability of continuous oscillation at high temperatures including the room temperature by designing and fabricating it such that, when two amounts α and β are defined by α.tbd.(R_t /T₀)I_th V_th and β.tbd.(R_t /T₀)R_S I_th², the point (α,β) exists in an area on the α-β plane surrounded by the straight line α=0, the straight line β=0 and the curve ((2ln t-1)/t, (1-ln t)/t²) having t as a parameter.

In particular, by using ZnMgSSe compound semiconductors among II-VI compound semiconductors as materials of the cladding layers and the optical waveguide layers, it is possible to realize a semiconductor laser capable of emitting, for example, blue to green light, and a semiconductor laser capable of emitting light in the near-ultraviolet range, depending on a material chosen for the active layer.

Embodiments of the invention are described below with reference to the drawings. In all the drawings of embodiments, identical or corresponding elements are labelled with the same reference numerals.

FIG. 3 shows a semiconductor laser according to a first embodiment of the invention. The semiconductor laser according to the first embodiment has an SCH (separated confinement heterostructure) structure.

As shown in FIG. 3, the semiconductor laser according to the first embodiment uses an n-type GaAs substrate 1 having the (100) orientation and doped with Si, for example, as an n-type impurity. Sequentially stacked on the n-type GaAs substrate 1 are an n-type ZnSe buffer layer 2 doped with, for example, Cl as an n-type impurity; an n-type Zn₁-p Mg_p S_q Se₁-q cladding layer 3 doped with, for example, Cl as an n-type impurity; an n-type ZnSe optical waveguide layer 4 doped with, for example, Cl as an n-type impurity; an active layer 5; a p-type ZnSe optical waveguide layer 6 doped with, for example N as a p-type impurity; a p-type Zn₁-p Mg_p S_q Se₁-q cladding layer 7 doped with, for example, N as a p-type impurity; a p-type ZnS_v Se₁-v layer 8 doped with, for example, N as a p-type impurity; and a p-type ZnSe contact layer 9 doped with, for example, N as a p-type impurity.

In this case, the p-type ZnSe contact layer 9 and an upper portion of the p-type ZnS_v Se₁-v layer 8 are patterned in the form of stripes. The width of each stripe portion is, for example, 5 μm.

Further provided on the p-type ZnS_v Se₁-v except the stripe portions is an insulating layer 10 in the form of, for example, a 300 nm thick alumina (Al₂ O₃) film. Formed on the stripe-shaped p-type ZnSe contact layer 9 and on the insulating layer 10 is a p-side electrode 11. The portion where the p-side electrode 11 contacts the p-type ZnSe contact layer 9 behaves as a flow path of the electric current. The p-side electrode 11 may be made, for example, as an Au/Pt/Pd electrode made by sequentially stacking a 10 nm thick Pd film, a 100 nm thick Pt film and a 300 nm thick Au film. On the other hand, an n-side electrode such as In electrode contacts the back surface of the n-type GaAs substrate 1.

The semiconductor laser according to the first embodiment are end-coated. More specifically, FIG. 4 is a cross-sectional view taken along a plane parallel to the cavity length direction of the semiconductor laser. As shown in FIG. 4, the front one of a pair of cavity end surfaces vertical to the cavity length direction, from which a laser beam is taken out, is coated with a multi-layered film composed of a 74 nm thick Al₂ O₃ film 13 and a 31 nm thick Si film 14, and the rear one of the pair of cavity end surfaces vertical to the cavity length direction, from which a laser beam is not taken out, is coated with a multilayered film made by stacking two cycles of 74 nm thick Al₂ O₃ films 13 and 31 nm thick Si films 14. The thickness of the multi-layered films composed of the Al₂ O₃ film(s) 13 and the Si film(s) 14 is chosen such that the optical distance obtained by multiplying it by its refraction factor equals 1/4 of the wavelength of oscillation of a laser beam. In this case, the refraction factor of the front end surface is 70%, and the refraction factor of the rear end surface is 95%.

In the first embodiment, the active layer 5 is preferably 2 nm-20 nm thick, and has a single quantum well structure including an i-type Zn₁-z Cd_z Se quantum well layer having the thickness of, for example, 9 nm. In this case, the n-type ZnSe optical waveguide layer 4 and the p-type ZnSe optical waveguide layer 6 behave as barrier layers.

The Mg composition ratio p of the n-type Zn₁-p Mg_p S_q Se₁-q cladding layer 3 and the p-type Zn₁-p Mg_p S_q Se₁-q cladding layer 7 is, for example, 0.09, the S composition ratio q thereof is, for example, 0.18, and their band gap E_q is about 2.94 eV at 77K. The n-type Zn₁-p Mg_p S_q Se₁-q cladding layer 3 and the p-type Zn₁-p Mg_p S_q Se₁-q cladding layer 7 having the Mg composition ratio p=0.09 and the S composition ratio q=0.18 are in lattice matching with GaAs. The Cd composition ratio z of the i-type Zn₁-z Cd_z Se quantum well layer of the active layer 5 is, for example, 0.19, and the band gap E_g is about 2.54 eV at 77K. In this case, the difference ΔE_g between the band gap E_g of the n-type Zn₁-p Mg_p S_q Se₁-q cladding layer 3 and the p-type Zn₁-p Mg_p S_q Se₁-q cladding layer 7 and the band gap E_g of the i-type Zn₁-z Cd_z Se quantum well layer forming the active layer 5 is 0.40 eV. Values of band gaps E_g at the room temperature can be obtained by subtracting 0.1 eV from the values of the band gaps E_g at 77K.

In this case, the thickness of the n-type Zn₁-p Mg_p S_q Se₁-q cladding layer 3 is, for example, 1.5 μm thick, and has the impurity concentration of, for example, 5×10¹7 cm-3 in N_D -N_A (N_D is the donor concentration, and N_A is the acceptor concentration). The n-type ZnSe waveguide layer 4 is, for example, 80 nm thick, and has the impurity concentration of, for example, 5×10¹7 cm-3 in N_D -N_A. The p-type ZnSe waveguide layer 6 is, for example, 80 nm thick, and has the impurity concentration of, for example, 5×10¹7 cm-3 in N_A -N_D. The p-type Zn₁-p Mg_p S_q Se₁-q cladding layer 7 is, for example, 0.8 μm thick, and has the impurity concentration of, for example, 2×10 cm in N_A -N_D. The p-type ZnS_v Se₁-v layer 8 is, for example, 0.8 μm thick, and has the impurity concentration of, for example, 8×10¹7 cm-3 in N_A -N_D. The p-type ZnSe contact layer 9 is, for example, 45 nm thick, and has the impurity concentration of 8×10¹7 cm-3 in N_A -N_D.

Since small lattice mismatching exists between ZnSe and GaAs, dislocation may occur due to the lattice mismatching during epitaxial growth of the n-type ZnSe buffer layer 2 and other layers thereon. To prevent this, the thickness of the n-type ZnSe buffer layer 2 is chosen to be sufficiently smaller than the critical thickness of ZnSe (-100 nm), that is, for example, 33 nm in the first embodiment.

The cavity length L of the semiconductor laser according to the first embodiment is chosen to be, for example, 640 μm, and the width in the direction vertical to the cavity length direction is chosen to be, for example, 400 μm.

In the first embodiment, the p-type ZnS_v Se₁-v layer 8 stacked on the p-type Zn₁-p Mg_p S_q Se₁-q layer 7 has two or more functions, depending on cases, among the function as the second p-type cladding layer added to the p-type Zn₁-p Mg_p S_q Se₁-q cladding layer 7, the function of providing lattice matching with the p-type Zn₁-p Mg_p S_q Se₁-q cladding layer 7, the function as a spacer layer for preventing a short circuit caused by a solder creeping up along the chip end surfaces when a laser chip is mounted on a heat sink, as will be described later, and other functions. Although the Mg composition ratio and the S composition ratio q of the p-type Zn₁-p Mg_p S_q Se₁-q cladding layer 7 must be taken into consideration, the S composition ratio v of the p-type ZnS_v Se₁-v layer 8 is chosen in the range of 0<v≦0.1 and preferably 0.06≦v≦0.08, and the optimum S composition ratio v for lattice matching with the p-type Zn₁-p Mg_p S_q Se₁-q cladding layer 7 is 0.06.

Stacking the p-type ZnS_v Se₁-v layer 8 on the p-type Zn₁-p Mg_p S_q Se₁-q cladding layer 7 provides the following various advantages. That is, when the p-type ZnS_v Se₁-v layer 8 is used as the second p-type cladding layer, it is possible to minimize the thickness of the second cladding layer 7 whose epitaxial growth is not so easy as a binary or ternary II-V compound semiconductor, thereby so much facilitating fabrication of the semiconductor laser. Further, if the thickness of the entire p-type cladding layer is identical, the resistance of the p-type cladding layer can be made lower in the arrangement in which the p-type cladding layer is composed of the p-type Zn₁-p Mg_p S_q Se₁-q cladding layer 7 and the p-type ZnS_v Se₁-v layer 8 than in the arrangement in which the p-type cladding layer consists of the p-type Zn₁-p Mg_p S_q Se₁-q cladding layer 7 alone. In particular, when the p-type Zn₁-p Mg_p S_q Se₁-q cladding layer 7 having the thickness of about 0.8 μm and N_A -N_D of about 2×10¹7 cm-3 and the p-type ZnS_v Se₁-v layer 8 having the thickness of about 0.8 μm and N_A -N_D of about 8×10¹7 cm-3, the resistance of the entire p-type cladding layer can be made sufficiently low without deteriorating the optical confinement characteristics and the carrier confinement characteristics.

If the p-type ZnSe contact layer 9 is directly stacked on the p-type Zn₁-p Mg_p S_q Se₁-q cladding layer 7, the crystalline quality would be deteriorated due to lattice mismatching between these layers. In the embodiment, however, since the p-type ZnS_v Se₁-v layer 8 having substantially the same lattice constant as that of the p-type Zn₁-p Mg_p S_q Se₁-q cladding layer 7 is stacked on the p-type Zn₁-p Mg_p S_q Se₁-q cladding layer and the p-type ZnSe contact layer 9 is stacked on the p-type ZnS_v Se₁-v layer 8, good crystalline qualities of the p-type ZnS_v Se₁-v layer 8 and the p-type ZnSe contact layer 9 can be realized.

Additionally, by providing the p-type ZnS_v Se₁-v layer 8 with a sufficient thickness, it can be effectively prevented that a solder used to mount a laser chip on a heat sink creeps up along the end surfaces of the laser chip and makes a short circuit between the p-side and the n-side. More specifically, as shown in FIG. 5, when a laser chip is mounted on a heat sink 41 in p-side down by orienting the p-side electrode 11 downward, no problem would arise if the solder 42 remains only between the laser chip and the heat sink 41 as shown by the solid line. Even if the solder creeps up, for example, in the form of lines, along the end surfaces of the laser chip as shown by the dash-and-dot lines because of a somewhat rough soldering, a sufficiently large thickness of the p-type ZnS_v Se₁-v layer 8 can prevent the solder 42 from creeping up along the end surfaces of the laser chip and reaching the n-type ZnSe optical waveguide layer 4 and the n-type Zn₁-p Mg_p S_q Se₁-q cladding layer 3 beyond the active layer 5. Typically, the creeping-up of the solder 42 can be terminated far below the active layer 5. As a result, the embodiment can prevent short circuiting of the p-side and the n-side of the laser chip during the mounting thereof, and thereby facilitating the mounting of the laser chip.

Next explained is a method of fabricating the semiconductor laser according to the first embodiment.

FIG. 6 shows a molecular beam epitaxy (MBE) apparatus used in the method of fabricating the semiconductor laser according to the first embodiment for epitaxially growing respective layers constituting the laser structure. As shown in FIG. 6, the MBE apparatus includes a plurality of molecular beam sources (K cells) 54, a substrate holder 55 for holding a substrate for epitaxial growth thereon, and an electron cyclotron resonance (ECR) plasma cell 56 which are contained in a vacuum container 53 which can be evacuated into ultrahigh vacuum by an ultrahigh evacuating apparatus 52 connected thereto via a gate valve 51.

To fabricate the semiconductor laser according to the first embodiment, an n-type GaAs substrate 1 is first set on a substrate holder 55 in the vacuum container 53 of the MBE apparatus shown in FIG. 6. The n-type GaAs substrate 1 is then heated to a temperature sufficiently higher than the growth temperature, for example, to 580°C, for cleaning the surface. After that, the temperature of the n-type GaAs substrate 1 is decreased to a predetermined epitaxial growth temperature, preferably in the range of 250°-300°C, more preferably in the range of 280°-300°C, specifically to 295°C, to start the epitaxial growth. That is, by using the MBE method, epitaxially grown in sequence on the n-type GaAs substrate 1 are the n-type ZnSe buffer layer 2, the n-type Zn₁-p Mg_p S_q Se₁-q cladding layer 3, the n-type ZnSe optical waveguide layer 4, the active layer 5 composed of the i-type Zn₁-z Cd_z Se quantum well layer, the p-type ZnSe optical waveguide layer 6, the p-type Zn₁-p Mg_p S_q Se₁-q cladding layer 7, the p-type ZnS_v Se₁-v layer 8, and the p-type ZnSe contact layer 9. In this case, it is possible to epitaxially grow these layers with good crystalline qualities, to hence suppress deteriorations such as a decrease in light output of the semiconductor laser and to obtain a high reliability.

The epitaxial growth by the MBE method uses Zn having the purity of 99.9999% as the source of Zn, Mg having the purity of 99.9% as the source of Mg, Zns having the purity of 99.9999% as the source of S, and Se having the purity of 99.9999% as the source of Se. Further, doping of Cl as the n-type impurity into the n-type ZnSe buffer layer 2, n-type Zn₁-p Mg_p S_q Se₁-q cladding layer 3 and n-type ZnSe optical waveguide layer 4 is done by using, for example, ZnCl₂ having the purity of 99.9999% as the dopant. On the other hand, doping of N as the p-type impurity to the p-type ZnSe optical waveguide layer 6, p-type Zn₁-p Mg_p S_q Se₁-q cladding layer 7 and p-type ZnSe contact layer 9 is done by irradiating N₂ plasma blasted by ECR, for example.

After that, a stripe-shaped resist pattern (not shown) with a predetermined width is formed on the p-type ZnSe contact layer 9, and the p-type ZnS_v Se₁-v layer 8 is etched up to an intermediate portion in its thickness direction by a wet etching method using the resist pattern as a mask. As a result, the p-type ZnSe contact layer 9 and an upper portion of the p-type ZnS_v Se₁-v layer 8 are patterned into stripes.

After that, while maintaining the resist pattern used by the etching, an Al₂ O₃ film is deposited on the entire surface by vacuum evaporation. Thereafter, the resist pattern and the Al₂ O₃ film thereon are removed (lift-off). As a result, an insulating layer 10 composed of the Al₂ O₃ is formed only on the p-type ZnS_v Se₁-v layer 8 in the areas other than the stripe portions.

Next formed on the entire surface including the stripe-shaped p-type ZnSe contact layer 9 and the insulating layer 10 is the p-side electrode composed of an Au/Pt/Pd electrode made by sequentially depositing Pd, Pt and Au films by vacuum evaporation. Then, after a heat treatment, if necessary, the p-side electrode 11 is brought into ohmic contact with the p-type ZnSe contact layer 9. Formed on the back surface of the n-type GaAs substrate 1 is the n-side electrode 12 such as In electrode.

After that, the n-type GaAs substrate 1 provided with the laser structure thereon is cleaved into bars having the width of 640 μm, for example, to thereby make cavity end surfaces. After that, by vacuum evaporation, a multi-layered film composed of an Al₂ O₃ film 13 and a Si film 14 is formed on the front end surface, and a multi-layered film made by repeating two cycles of Al₂ O₃ films 13 and Si films 14 is formed on the rear end surface. After such end coating, the bar is cleaved into chips having the width of 400 μm, and the chips are packaged.

FIG. 7 shows a result of measurement of characteristics, in term of the light output relative to the injected current, which was conducted with the semiconductor laser according to the first embodiment at the room temperature (296K) by applying a continuous flow of the injected current and by applying a pulsating flow of the injected current. The measurement was conducted by mounting the laser chip in p-side down on the heat sink 41 of copper as shown in FIG. 5. As apparent from FIG. 7, the threshold current I_th with a continuous flow of the injected current is about 45 mA which corresponds to the threshold current density J_th of about 1.5 kA/cm². On the other hand, the threshold current I_th with a pulsating flow of the injected current is about 42 mA. Note that the measurement of the characteristics in term of the light output relative to the current with a continuous flow of the injected current was conducted by increasing the injected current from 0 to 100 mA at the speed of 500 mA/second. On the other hand the measurement of the characteristics in term of the light output relative to the current with a pulsating flow of the injected current was conducted by applying the injected current with the pulse width of I_th =(pulse) I_th (pulse)=48 mA=0.048A, V_th =8V, R_S =12Ω, R_t =23K/W, and T₀ 150K. In FIG. 2, (α, β)=(0.059, 0.0042) has been plotted with a black point. It is apparent from FIG. 2 that (0.059, 0.0042) also certainly exists in the area D.

The second embodiment, like the first embodiment, can realize a semiconductor laser capable of emitting green light by continuous oscillation at the room temperature and having the SCH structure with a low threshold current density. This semiconductor laser generates only a small amount of heat during operation, and can be fabricated easily. In particular, with the second embodiment, since the p-type ZnTe/ZnSe MQW layer 15 and the p-type ZnTe contact layer 16 are stacked on the p-type ZnSe contact layer 9 such that the p-side electrode contacts with the p-type ZnTe contact layer 16, generation of heat from the semiconductor laser during its operation can be significantly reduced, and a significant amount of voltage applied for oscillation of the laser can be reduced.

Having described specific embodiments of the present invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or the spirit of the invention as defined in the appended claims.

For example, the n-type ZnSe optical waveguide layer 4 and the p-type ZnSe optical waveguide layer 6 used in the first and second embodiments may be replaced by i-type ZnSe optical waveguide layers. From the viewpoint of lattice matching, it is desirable to use an n-type ZnS_u Se₁-u and a p-type ZnS_u Se₁-u particularly of u=0.06 in lieu of the n-type ZnSe optical waveguide layer 4 and the p-type ZnSe optical waveguide layer 6.

Although the first and second embodiments irradiate N₂ plasma generated by ECR for doping N as a p-type impurity into the p-type ZnSe optical waveguide layer 6, p-type Zn₁-p Mg_p S_q Se₁-q cladding layer 7, p-type ZnS_v Se₁-v layer 8, p-type ZnSe contact layer 9, and p-type ZnTe contact layer 16, doping of N may be done, for example, by irradiating N₂ excited by a high frequency plasma.

Further, the GaAs substrate used in the first and second embodiments as a compound semiconductor substrate may replaced with, for example, a GaP substrate.

Although the first and second embodiments have been described as applying the invention to semiconductor lasers having an SCH structure, the invention may be applied to a semiconductor laser having a DH (double heterostructure) structure.

The same technical concept as the present invention can also be applied to semiconductor lasers using III-V compound semiconductors. For example, it may be applied to semiconductor lasers using GaN compound semiconductor as the material of the active layer and using an AlGaN compound semiconductor as the material of the cladding layers, and capable of emitting blue light, or may be applied to semiconductor lasers using an AlGaInP compound semiconductor as the material of the active layer and using an AlGaInP compound semiconductor having a larger band gap than the AlGaInP compound semiconductor of the active layer as the material of the cladding layer, and capable of emitting green to yellow-green light.

Further, the same technical concept can also be applied to semiconductor lasers using chalcopyrite semiconductors. Calcopyrite semiconductors usable are, for example, those of a first kind including a group I element, a group III element and a group VI element in the ratio of atoms of 1:1:2, specifically such as CuInSe₂, CuAlSe₂, or the like; those of a second kind including a group II element, a group IV element and a group V element in the ratio of atoms of 1:1:2, specifically such as ZnSiP₂, ZnSiAs₂, ZnGeP₂, ZnGeAs₂, ZnSnP₂, ZnSnAs₂, CdSiP₂, CdSiAs₂, CdGeP₂, CdGeAs₂, CdSnP₂, CdSnAs₂, or the like; and those of a third kind including a group II element, a group III element and a group VI element in the ratio of atoms of 1:2:4, specifically such as ZnGa₂ S₄, ZnIn₂ S₄, CdGa₂ S₄, CdIn₂ S₄, or the like.

As described above, the invention can realize a semiconductor laser using a II-VI compound semiconductor as the material of its cladding layers and having the capability of continuous oscillation at high temperatures including the room temperature.

Claims

1. A semiconductor laser comprising:

a first cladding layer of a first conduction type;

an active layer stacked on said first cladding layer; and

a second cladding layer of a second conduction type stacked on said active layer, wherein:

said first cladding layer, said active layer and said second cladding layer are made of II-VI compound semiconductors, and

pulse oscillation occurs with characteristics of a threshold current i_th (A), a threshold voltage v_th (v) of a diode composed of said first cladding layer, said active layer and said second cladding layer, a differential resistance r_s (Ω) of said diode after the rising, a thermal resistance r_t (K/W) and a characteristic temperature t₀ (K), and wherein:

when two amounts α and β are defined by:

α.tbd.(r_t /t₀)i_th v_th

β.tbd.(r_t /t₀)r_s i_th²

continuous oscillation occurs at room temperature when the operating point (α,β) exists in an area on the α-β plane surrounded by the straight line α=O, the straight line β=0, and the curve ((2ln t-1)/t, (1-ln t)/t²) having t as a parameter.

2. The semiconductor laser according to claim 1 including a first optical waveguide layer between said first cladding layer and said active layer and including a second optical waveguide layer between said second cladding layer and said active layer, said first optical waveguide layer and said second optical waveguide layer being made of a II-VI compound semiconductor.

3. The semiconductor laser according to claim 1 wherein said II-VI compound semiconductor forming said first cladding layer and said second cladding layer is a ZnMgSSe compound semiconductor.

4. A semiconductor laser comprising:

a first cladding layer of a first conduction type;

an active layer stacked on said first cladding layer; and

a second cladding layer of a second conduction type stacked on said active layer, wherein:

said first cladding layer, said active layer and said second cladding layer are made of III-v GaN type compound semiconductor, and

pulse oscillation occurs with characteristics of a threshold current i_th (A), a threshold voltage v_th (v) of a diode composed of said first cladding layer, said active layer and said second cladding layer, a differential resistance r_s (Ω) of said diode after the rising, a thermal resistance r_t (K/W) and a characteristic temperature t₀ (K), and wherein:

when two amounts α and β are defined by:

α.tbd.(r_t /t₀)i_th v_th

β.tbd.(r_t /t₀)r_s i_th²

continuous oscillation occurs at room temperature when the operating point (α,β) exists in an area on the α-β plane surrounded by the straight line α=0, the straight line β=0, and the curve ((2ln t-1)/t, (1-ln t)/t²) having t as a parameter.

5. The semiconductor laser according to claim 4 including a first optical waveguide layer between said first cladding layer and said active layer and including a second optical waveguide layer between said second cladding layer and said active layer, said first optical waveguide layer and said second optical waveguide layer being made of a III-v compound semiconductor.

6. A method of fabricating a laser, said method comprising the steps of:

forming a semiconductor laser structure by:

selecting first, second and third compositions of II-VI compound semiconductors for a first cladding layer, an active layer and second cladding layer, respectively, such that

pulse oscillating occurs with characteristics of a threshold current i_th (A), a threshold voltage v_th (v) of a diode composed of said first cladding layer, said active layer and said second cladding layer, a differential resistance r_s (Ω) of said diode, a thermal resistance r_t (K/W) and a characteristic temperature t₀ (K), and wherein

when two amounts α and β are defined by:

α.tbd.(r_t /t₀)i_th v_th

β.tbd.(r_t /t₀)r_s i_th²

the point (α,β) exists in an area on the α-β plane surrounded by the straight line β=0, the straight line β=0, and the curve ((2ln t-1)/t, (1-ln t)/t²) having t as a parameter; and then:

forming the first cladding layer of a first conduction type from said first composition of II-VI compound semiconductors;

forming the active layer stacked on said first cladding layer from said second composition of II-VI compound semiconductors; and

forming the second cladding layer of a second conduction type stacked on said active layer from said third composition of II-VI compound semiconductors.

7. The method of fabricating a laser as claimed in claim 6, wherein said step of forming a semiconductor laser structure further comprises the steps of:

forming a first optical wave guide layer between said first cladding layer and said active layer; and

forming a second optical wave guide layer between said second cladding layer and said active layer, said first optical wave guide layer and said second optical wave guide layer being made of a II-VI compound semiconductor.

8. The method of fabricating a laser as claimed in claim 6, wherein said II-VI compound semiconductor forming said first cladding layer and said second cladding layer is a ZnMgSSe compound semiconductor.

9. A method of fabricating a laser, said method comprising the steps of:

forming a semiconductor laser structure by:

selecting first, second and third compositions of III-v compound semiconductors for a first cladding layer, an active layer and a second cladding layer, respectively, such that

pulse oscillation occurs with characteristics of a threshold current i_th (A), a threshold voltage v_th (v) of a diode composed of said first cladding layer, said active layer and said second cladding layer, a differential resistance r_s (Ω) of said diode, a thermal resistance r_t (K/W) and a characteristic temperature t₀ (K), and wherein

when two amounts α and β are defined by:

α.tbd.(r_t /t₀)i_th v_th

β.tbd.(r₁ /t₀)r_s i_th²

the point (α,β) exists in an area on the α-β plane surrounded by the straight line α=0, the straight line β=0, and the curve ((2ln t-1)/t, (1-ln t)/t²) having t as a parameter; and then:

forming the first cladding layer of a first conduction type from said first composition of III-v compound semiconductors;

forming the active layer stacked on said first cladding layer from said second composition of III-v compound semiconductors; and

forming the second cladding layer of a second conduction type stacked on said active layer from said third composition of III-v compound semiconductors.

10. The method of fabricating a laser as claimed in claim 9, wherein said step of forming a semiconductor laser structure further comprises the steps of:

forming a first optical wave guide layer between said first cladding layer and said active layer; and

forming a second optical wave guide layer between said second cladding layer and said active layer, said first optical wave guide layer and said second optical wave guide layer being made of a III-v compound semiconductor.