SEMICONDUCTORS STRUCTURES USING A GROUP III-NITRIDE QUATERNARY MATERIAL SYSTEM WITH REDUCED PHASE SEPARATION AND METHOD OF FABRICATION

Abstract: A group III-nitride quaternary material system and method is disclosed for use in semiconductor structures, including laser diodes, transistors, and photodetectors, which reduces or eliminates phase separation and provides increased emission efficiency. In an exemplary embodiment the semiconductor structure includes a first InGaAIN layer of a first conduction type formed substantially without phase separation, an InGaAIN active layer substantially without phase separation, and a third InGaAIN layer of an opposite conduction type formed substantially without phase separation.

Description: SEMICONDUCTORS STRUCTURES USING A GROUP III-NITRIDE QUATERNARY MATERIAL SYSTEM WITH REDUCED PHASE SEPARATION AND METHOD OF FABRICATION 15 20 SPECIFICATION 25 FIELD OF THE INVENTION This application relates to semiconductor structures and processes, and particularly relates to group Ill-nitride materials systems and methods such as might 30 be used in blue laser diodes. BACKGROUND OF THE INVENTION The development of the blue laser light source has heralded the next 35 generation of high density optical devices, including disc memories, DVDs, and so on. Figure 1 shows a cross sectional illustration of a prior art semiconductor laser devices. (S. Nakamura, MRS BULLETIN, Vol. 23, No. 5, pp. 37-43, 1998.) On a sapphire substrate 5, a gallium nitride (GaN) buffer layer 10 is formed, followed by an n-type GaN layer 15, and a 0.1 μm thick silicon dioxide (Si0 2 ) layer 20 which is patterned to form 4 μm wide stripe windows 25 with a periodicity of 12 μm in the GaN<1 -100> direction. Thereafter, an n-type GaN layer 30, an n-type indium gallium nitride (ln 0 .,Ga 0 9 N) layer 35, an n-type aluminum gallium nitride (Al 0 14 Ga 086 N)/GaN MD-SLS (Modulation Doped Strained-Layer Superlattices) cladding layer 40, and an n-type GaN cladding layer 45 are formed. Next, an ln 002 Ga 0 98 N/ln 0 15 Ga 085 N MQW (Multiple Quantum Well) active layer 50 is formed followed by a p-type AI 0 2 Ga 0 8 N cladding layer 55, a p-type GaN cladding layer 60, a p-type Al 0 Λ 4 Ga 086 N/GaN MD- SLS cladding layer 65, and a p-type GaN cladding layer 70. A ridge stripe structure is formed in the p-type Al 0 1 4 Ga 0 86 N/GaN MD-SLS cladding layer 55 to confine the optical field which propagates in the ridge waveguide structure in the lateral direction. Electrodes are formed on the p-type GaN cladding layer 70 and n-type GaN cladding layer 30 to provide current injection. In the structure shown in Figure 1 , the n-type GaN cladding layer 45 and the p-type GaN 60 cladding layer are light-guiding layers. The n-type AI 0 1 4 Ga 0 86 N/GaN MD-SLS cladding layer 40 and the p-type Al 0 < 4 Ga 0 86 N/GaN MD-SLS cladding layer 65 act as cladding layers for confinement of the carriers and the light emitted from the active region of the InGaN MQW layer 50. The n-type ln 0 .,Ga 0 9 N layer 35 serves as a buffer layer for the thick AIGaN film growth to prevent cracking. By using the structure shown in Figure 1 , carriers are injected into the InGaN MQW active layer 50 through the electrodes, leading to emission of light in the wavelength region of 400 nm. The optical field is confined in the active layer in the lateral direction due to the ridge waveguide structure formed in the p-type Al 0 1 4 Ga 0 86 N/GaN MD-SLS cladding layer 65 because the effective refractive index under the ridge stripe region is largerthan that outside the ridge stripe region. On the other hand, the optical field is confined in the active layer in the transverse direction by the n-type GaN cladding layer 45, the n-type Al 0 14 Ga 0 86 N/GaN MD-SLS cladding layers 40, the p-type GaN cladding layer 60, and the p-type Al 0 14 Ga 0 86 N/GaN MD-SLS cladding layer 55 because the refractive index of the of the active layer is larger than that of the n-type GaN cladding layer 45 and the p-type GaN cladding layer 60, the n-type Al 0 14 Ga 0 86 N/GaN MD-SLS Iayer40, and the p-type Al 0 14 Ga 0 86 N/GaN MD-SLS cladding layer 60. Therefore, fundamental transverse mode operation is obtained. However, for the structure shown in Figure 1 , it is difficult to reduce the defect density to the order of less than 10 8 cm "2 , because the lattice constants of AIGaN, InGaN, and GaN differ sufficiently different from each other that defects are generated in the structure as a way to release the strain energy whenever the total thickness of the n-type ln 0 1 Ga o g N layer 35, the ln 002 Ga 098 N/ln 0 15 Ga 085 N MQW active layer 50, the n-type AI 0 14 Ga 0 86 N/GaN MD-SLS cladding layer 40, the p-type Al 0 14 Ga 086 N/GaN MD-SLS cladding layer 65, and the p-type AI 02 Ga 0 8 N cladding layer 55 exceeds the critical thickness. The defects result from phase separation and act as absorption centers for the lasing light, causing decreased light emission efficiency and increased threshold current. The result is that the operating current becomes large, which in turn causes reliability to suffer. Moreover, the ternary alloy system of InGaN is used as an active layer in the structure shown in Figure 1. In this case, the band gap energy changes from 1.9 eV for InN to 3.5 eV for GaN. Therefore, ultraviolet light which has an energy level higher than 3.5 eV cannot be obtained by using an InGaN active layer. This presents difficulties, since ultraviolet light is attractive as a light source for the optical pick up device in, for example, higher density optical disc memory systems and other devices. To better understand the defects which result from phase separation in conventional ternary materials systems, the mismatch of lattice constants between InN, GaN, and AIN must be understood. The lattice mismatch between InN and GaN, between InN and AIN, and between GaN and AIN, are 1 1 .3%, 13.9%, and 2.3%, respectively. Therefore, an internal strain energy accumulates in an InGaAIN layer, even if the equivalent lattice constant is the same as that of the substrate due to the fact that equivalent bond lengths are different from each other between InN, GaN, and AIN. In order to reduce the internal strain energy, there is a compositional range which phase separates in the InGaAIN lattice mismatched material system, where In atoms, Ga atoms, and Al atoms are inhomogeneously distributed in the layer. The result of phase separation is that In atoms, Ga atoms, and Al atoms in the InGaAIN layers are not distributed uniformly according to the atomic mole fraction in each constituent layer. In turn, this means the band gap energy distribution of any layerwhich includes phase separation also becomes inhomogeneous. The band gap region of the phase separated portion acts disproportionately as an optical absorption center or causes optical scattering for the waveguided light. As noted above, a typical prior art solution to these problems has been to increase drive current, thus reducing the life of the semiconductor device. As a result, there has been a long felt need for a semiconductor structure which minimizes lattice defects due to phase separation and can be used, for example, as a laser diode which emits blue or UV light at high efficiency, and for other semiconductor structures such as transistors. SUMMARY OF THE INVENTION The present invention substantially overcomes the limitations of the prior art by providing a semiconductor structure which substantially reduces defect densities by materially reducing phase separation between the layers of the structure. This in turn permits substantially improved emission efficiency. To reduce phase separation, it has been found possible to provide a semiconductor device with InGaAIN layers having homogeneous In content, Al content, Ga content distribution in each layer. In a light emitting device, this permits optical absorption loss and waveguide scattering loss to be reduced, resulting in a high efficiency light emitting device. A quaternary material system such as InGaAIN has been found to provide, reproducibly, sufficient homogeneity to substantially reduce phase separation where the GaN mole fraction, x, and the AIN mole fraction, y, of all the constituent layers in the semiconductor structure satisfy the condition that x + 1 .2y nearly equals a constant value. A device according to the present invention typically includes a first layer of InGaAIN material of a first conductivity, an InGaAIN active layer, and a layer of InGaAIN material of an opposite conductivity successively formed on one another. By maintaining the mole fractions essentially in accordance with the formula x + 1 .2y equals a constant, for example on the order of or nearly equal to one, the lattice constants of the constituent layers remain substantially equal to each other, leading to decreased generation of defects. In an alternative embodiment, the semiconductor structure is fabricated essentially as above, using a quaternary materials system to eliminate phase separation and promote homogeneity across the layer boundaries. Thus, as before, the first cladding layer is a first conduction type and composition of InGaAIN, the active layer is InGaAIN of a second composition, and the second cladding layer is an opposite conduction type of InGaAIN having the composition of the first layer. However, in addition, the second cladding layer has a ridge structure. As before, the optical absorption loss and waveguide scattering loss is reduced, leading to higher efficiencies, with added benefit that the optical field is able to be confined in the lateral direction in the active layer under the ridge structure. This structure also permits fundamental transverse mode operation. In a third embodiment of the invention, suited particularly to implementation as a laser diode, the semiconductor structure comprises a first cladding of a first conduction type of an In^.^Ga^AI^N material, an active layer of an ln 1 _ x2.y2 Ga x2 Al y2 N material, and a second cladding layer of an opposite conduction type of an In^. y3 Ga x3 Al y3 N material, each successively formed on the prior layer. In such a materials system, x1 , x2, and x3 define the GaN mole fraction and y1 , y2, and y3 define the AIN mole fraction and x1 , y1 , x2, y2, x3, and y3 have a relationship of 0 < x1 + y1 <1 , 0 < x2 + y2 <1 , 0 < x3 + y3 <1 , 1<= x1/0.80 + y1/0.89, 1<= x2/0.80 + y2/0.89, 1 <= X3/0.80 + y3/0.89, Eg lnN (1-x1-y1 ) + Eg GaN x1 + Eg AIN y1 > Eg lnN (1-x2-y2) + Eg GaN x2 + Eg AIN y2, and Eg lnN (1-x3-y3) + Eg GaN x3 + Eg AIN y3 > Eg lnN (1-x2-y2) + Eg GaN x2 + Eg AIN y2, where Eg lnN , Eg GaN , and Eg A , N are the band gap energy of InN, GaN, and AIN, respectively. To provide a reproducible semiconductor structure according to the above materials system, an exemplary embodiment of InGaAIN layers have Ga content, x, and Al content, y, which satisfy the relationship 0 < x + y <1 and 1 <= x/0.80 + y/0.89. As before, this materials system permits reduction of the optical absorption loss and the waveguide scattering loss, resulting in a high efficiency light emitting device. Moreover, the band gap energy of the InGaAIN of an active layer becomes smaller than that of the first cladding layer and the second cladding layer when x1 , y1 , x2, y2, x3, and y3 have a relationship of 0 < x1 + y1 <1 , 0 < x2 + y2 <1 , 0 < x3 + y3 <1 , 1 <= xl/0.80 + y1 /0.89, 1 <= x2/0.80 + y2/0.89, 1 <= x3/0.80 + y3/0.89, Eg lnN (1 -x1-y1 ) + Eg GaN x1 + Eg AIN y1 > Eg lnN (1 -x2-y2) + Eg GaN x2 + Eg AlN y2, and Eg lnN (1-x3-y3) + Eg GaN x3 + Eg AIN y3 > Eg lnN (1 -x2-y2) + Eg GaN x2 + Eg AIN y2. Under these conditions, the injected carriers are confined in the active layer. In at least some embodiments, it is preferable that the third light emitting device has an InGaAIN single or multiple quantum well active layer whose GaN mole fraction, xw, and AIN mole fraction, yw, of all the constituent layers satisfy the relationship of 0 < xw + yw <1 and 1 <= x/0.80 + y/0.89. One of the benefits of the foregoing structure is to reduce the threshold current density of a laser diode. This can be achieved by use of a single or multiple quantum well structure, which reduces the density of the states of the active layer. This causes the carrierdensity necessary for population inversion to become smaller, leading to a reduced or low threshold current density laser diode. It is preferred that in the third light emitting device, the condition of xs + 1 .2ys nearly equals to a constant value -- on the order of or near one -- is satisfied, wherein xs and ys are the GaN mole fraction and the AIN mole fraction, respectively in each the constituent layers. As before, this causes the lattice constants of the each constituent layers to be nearly equal to each other, which in turn substantially minimizes defects due to phase separation In a fourth embodiment of the present invention, the semiconductor structure may comprise a first cladding layer of a first conduction type of a material ln.,. x1 . y1 Ga x1 Al y1 N, an In^.^Ga^AI^N active layer, and a second cladding layer of an opposite conduction type of a material In^^Ga^AI^N, each successively formed one upon the prior layer. In addition, the second cladding layer has a ridge structure. For the foregoing materials system, x1 , x2, and x3 define the GaN mole fraction, y1 , y2, and y3 define the AIN mole fraction, and x1 , y1 , x2, y2, x3, and y3 have a relationship of 0 < x1 + y1 <1 , 0 < x2 + y2 <1 , 0 < x3 + y3 <1 , 1<= x1/0.80 + y1/0.89, 1 <= X2/0.80 + y2/0.89, 1 <= x3/0.80 + y3/0.89, Eg, nN (1 -x1-y1 ) + Eg GaN x1 + Eg AIN y1 > Eg lnN (1-x2-y2) + Eg GaN x2 + Eg AIN y2, and Eg lnN (1-x3-y3) + Eg GaN x3 + Eg AlN y3 > Eg lnN (1- x2-y2) + Eg GaN x2 + Eg AlN y2, where Eg* nN , Eg GaN , and Eg AIN are the bandgap energy of InN, GaN, and AIN, respectively. As with the prior embodiments, each of the InGaAIN layers have a homogeneous In content, Al content, and Ga content distribution, which can be obtained reproducibly when Ga content, x, Al content, y, of each InGaAIN layer satisfies the relationship 0 < x + y <1 and 1 <= x/0.80 + y/0.89. The band gap energy of the InGaAIN active layer becomes smaller than that of the first cladding layer and the second cladding layer when x1 , y1 , x2, y2, x3, and y3 have a relationship of 0 < x1 + y1 <1 , 0 < x2 + y2 <1 , 0 < x3 + y3 <1 , 1 <= x1/0.80 + yl/0.89, 1 <= X2/0.80 + y2/0.89, 1 <= X3/0.80 + y3/0.89, Eg lnN (1 -x1 -y1 ) + Eg GaN x1 + Eg AIN y1 > Eg lnN (1 -x2-y2) + Eg GaN x2 + Eg AIN y2, and Eg lnN (1-x3-y3) + Eg GaN x3 + Eg AIN y3 > Eg lnN (1-x2-y2) + Eg GaN x2 + Eg A!N y2. Similar to the prior embodiments, the injected carriers are confined in the active layer and the optical field is confined in the lateral direction in the active layer under the ridge structure, producing a fundamental transverse mode operation. Also similar to the prior embodiments, the fourth embodiment typically includes an InGaAIN single or multiple quantum well active layer whose GaN mole fraction, xw, and AIN mole fraction, yw of all the constituent layers satisfy the relationship of 0 < xw + yw <1 and 1 <= x/0.80 + y/0.89. Also, the condition xs + 1 .2ys nearly equals to a constant value on the order of or near one is typically satisfied, where xs and ys are the GaN mole fraction and the AIN mole fraction, respectively in each constituent layer. Similar parameters apply for other substrates, such as sapphire, silicon carbide, and so on. The foregoing results may be achieved with conventional processing temperatures and times, typically in the range of 500°C to 1000°C. See "Growth of high optical and electrical quality GaN layers using low-pressure metalorganic chemical vapor deposition," Appl. Phys. Lett. 58 (5), 4 February 1991 p. 526 et seq. The present invention may be better appreciated by the following Detailed Description of the Invention, taken together with the attached Figures. FIGURES Figure 1 shows a prior art laser diode structure using a conventional ternary materials system. Figure 2 shows in cross-sectional view a semiconductor structure according to a first embodiment of the invention. Figure 3 shows a graph of the light-current characteristics of a laser diode according to the structure of Figure 1. Figure 4 shows an exemplary series of the fabrication steps for a semiconductor structure in accordance with a first embodiment of the invention. Figure 5 shows in cross-sectional view a semiconductor structure according to a second embodiment. Figure 6 shows a graph of the light-current characteristics of a laser diode according to the structure of Figure 4. Figure 7 shows an exemplary series of the fabrication steps for a semiconductor structure in accordance with the first embodiment of the invention. Figure 8 is a cross-sectional illustration of a semiconductor laser diode of the third embodiment. Figure 9 shows the light-current characteristics of the laser diode of the third embodiment. Figure 10 shows a series of the fabrication steps of a semiconductor laser diode in one experiment example of the third embodiment. Figure 1 1 is a cross-sectional illustration of a semiconductor laserdiode of the fourth embodiment. Figure 12 shows the light-current characteristics of the laser diode of the fourth embodiment. Figure 13 shows a series of the fabrication steps of a semiconductor laser diode in one experiment example of the fourth embodiment. Figure 14 shows in plot form the boundary between the phase separation region and the region without phase separation at various growth temperatures. Figure 15 shows the content choice region of Ga content and Al content in InGaAIN to avoid phase separation at a growth temperature below approximately 1000 °C. Figure 16 shows the content choice line of Ga content and Al content in InGaAIN to avoid phase separation at a growth temperature below approximately 1000 °C which, at the same time, creates a lattice constant of InGaAIN substantially equivalent to that of GaN. Figures 17A and 17B show representations of bipolar and FET transistors constructed in accordance with the materials system of the present invention. Figure 18 shows an implementation of the presention invention as a phototransistor. Figure 19 shows an implementation of the present invention as a photodiode. DETAILED DESCRIPTION OF THE INVENTION Referring first to Figure 2, shown therein in cross-sectional view is a semiconductor structure according to a first embodiment of the invention. For purposes of illustration, the semicoductor structure shown in many of the Figures will be a laser diode, although the present invention has appliction to a number of device types. A diode of the first embodiment. With particular reference to Figure 1 , an n-type GaN substrate 100 is provided and an n-type GaN first cladding layer 105 (typically 0.5μm thick) is formed thereon. Thereafter, a second cladding layer 1 10, typically of an n-type ln 0 05 Ga 0 75 AI 0 2 N material which may be on the order of 1 .5 μm thick, is formed thereon, followed by a multiple quantum well active layer 1 15 which in an exemplary arrangement may comprise three quantum well layers of ln 0 01 Ga 0 96 AI 003 N material on the order of 35A thick together with four barrier layers of ln 0 02 Ga 0 85 AI 0 13 N material on the order of 35A thick, arranged as three pairs. Next, a third cladding layer 120 of a p-type ln 0 05 Ga 0 75 AI 0 2 N( typically on the order of 1 .5 mm thick) is formed, followed by a p-type GaN fifth cladding layer 125 (on the order of 0.5μm thick). A Si0 2 layer 130 having one stripe like window region 135 (3.0μm width) is formed on the p-type GaN fourth cladding layer 125. A first electrode 140 is formed on the n-type GaN substrate 100, while a second electrode 145 is formed on the Si0 2 layer 130 and the window region 135. In order to emit ultra violet light with a wavelength range of 350 nm from the active layer 1 15, the InN mole fraction, the GaN mole fraction, and the AIN mole fraction of the well layer are set to be 0.01 , 0.96, and 0.03, respectively. To avoid defects due to phase separation, the lattice constants of the various constituent layers are matched to each other by setting the GaN mole fraction, x, and the AIN, y, in each of the layers to meet the condition x + 1 .2y nearly equals a constant value. In an exemplary embodiment, the constant value is set to nearly one, for example 1 +0.1 , although most embodiments will be in the range 1 +0.05. By proper selection of materials, the band gap energy of the n-type second cladding layer 1 10 and the p-type third cladding layer 120 are larger than that of the 3 pairs of multiple quantum well active layers 1 15. This confines the injected carriers from the n-type second cladding layer 1 10 and p-type third cladding layer 120 within the active layer 1 15, where the carriers recombine to lead to the emission of ultraviolet light. In addition, the refractive index of the n-type second cladding layer 5 1 10 and the p-type third cladding layer 120 are smaller than that of the multiple quantum well active layer 1 15, which confines the optical field in the transverse direction. Because the injected current from the electrode 145 is confined to flow through the window region 135, the region in the active layer 1 15 under the widow 0 region 135 is activated strongly. This causes the local modal gain in the active layer under the window region 6a to be higherthan the local modal gain in the active layer under the Si0 2 layer. Therefore, a gain guided waveguide mechanism, leading to a lasing oscillation, is able to be formed in the structure of the first embodiment. Figure 3 shows a plot of the emitted light versus drive current for a laser diode 5 constructed in accordance with the first embodiment. The laser diode is driven with a pulsed current with a duty cycle of 1 %. The threshold current density is found to be 5.5 kA/cm 2 . Figures 4A-4D show, in sequence, a summary of the fabrication steps necessary to construct an exemplary laser diode according to the first embodiment. 0 Since the structure which results from Figures 4A-4D will resemble that shown in Figure 2, like reference numerals will be used for elements whenever possible. With reference first to Figure 4A, an n-type GaN substrate 100 is provided, on which is grown an n-type GaN first cladding layer 105. The first cladding layer 105 is typically on the order of 0.5μm thick. Thereafter, an n-type ln 005 Ga 0 75 AI 0 2 N second cladding 5 layer 1 10 is formed, typically on the order of 1 .5 μm thick. Next, a multiple quantum well active layer 1 15 is formed by creating three quantum wells comprised of three layers of ln 001 Ga 0 96 AI 003 N material each on the order of 35A thick, together with four barrier layers of ln O O2 Ga 0 85 AI 0 13 N material on the order of 35A thick. A third cladding layer 120 of p-type ln 005 Ga 0 75 AI 0 2 N material, 0 on the order of 1 .5 μm thick, is then formed, after which is formed a fourth cladding layer 125 of a p-type GaN on the order of 0.5μm thick. Each of the layers is typically formed by eitherthe Metal Organic Chemical Vapor Deposition (MOCVD) method or the Molecular Beam Epitaxy (MBE) method. Then, as shown in Figure 4B, a silicon dioxide (SiO 2 ) layer 130 is formed on 5 the p-type GaN fourth cladding layer 125, for example by the Chemical Vapor Deposition (CVD) method. Using photolithography and etching or any other suitable method, a window region 135 is formed as shown in Figure 4C. The window region 135 may be stripe-like in at least some embodiments. Finally, as shown in Figure 4D, a first electrode 140 and a second electrode 145 are formed on the n-type GaN substrate 100 and on the SiO 2 layer 130, respectively, by evaporation or any other suitable process. Referring next to Figure 5, a second embodiment of a semiconductor structure in accordance with the present invention may be better appreciated. As with the first embodiment, an exemplary application of the second embodiment is the creation of a laser diode. The structure of the second embodiment permits a waveguide mechanism to be built into the structure with a real refractive index guide. This provides a low threshold current laser diode which can operate with a fundamental transverse mode. Continuing with reference to Figure 5, for ease of reference, like elements will be indicated with like reference numerals. On an n-type GaN substrate 100, a first cladding layer 105 is formed of an n-type GaN on the order of 0.5μm thick. Successively, an n-type second cladding layer 1 10 is formed of ln 0 05 Ga 0 75 AI 0 2 N material on the order of 1 .5 μm thick. Thereafter, a multiple quantum well active layer 1 15 is formed comprising three well layers of ln 0 O1 Ga 0 96 AI 0 03 N material on the order of 35A thick together with four barrier layers of ln 002 Ga 0 85 AI 0 13 N material, also on the order of 35λ thick. Next, a third, p-type cladding layer 120 formed of ln 0 05 Ga 0 75 AI 0 2 N material on the order of 1 .5 μm thick is formed. Thereafter, a p-type GaN fourth cladding layer 125 on the order of 0.5μm thick is formed over the ridge structure 500 of the third cladding layer 120. The third and fourth cladding layers are then partially removed to create a ridge structure 500. A silicon dioxide (Si0 2 ) layer 130 is then formed over over the fourth cladding layer 125 as well as the remaining exposed portion of the third cladding layer 120. A window region 135, which may be stripe-like on the order of 2.0μm width, is formed through the Si0 2 layer above the fourth and third cladding layers 125 and 120, respectively. As with the first embodiment, a first electrode 140 is formed on the n-type GaN substrate 100 and a second electrode 145 is formed on the Si0 2 layer 130 and the window region 135. As with the first embodiment, in order to emit ultra violet light with a wavelength in the range of 350 nm from the active layer 14, the mole fractions of InN, GaN, and AIN in the well layer are set to be 0.01 , 0.96, and 0.03, respectively. Likewise, in order to match the lattice constants of each of the constituent layers to avoid defects due to phase separation, the GaN mole fraction, x, and the AIN, y, of all the layers satisfy the condition of x + 1 .2y nearly equals a constant value. As in the first embodiment, x + 1 .2y set to be nearly equals 1 so that the equivalent lattice constants of each layers become nearly equal to the lattice constant of GaN. Likewise, the band gap energy of the cladding layers is maintained larger than the band gap energy for the active layer, allowing the emission of ultraviolet light. Similarly the refractive index of the materials is as discussed in connection with the first embodiment, permitting the optical field to be confined in the transverse direction. Similar to the operation of the first embodiment, the region of the active layer 1 15 underthe window region 135 is activated strongly because of the constraints on the injected current by the SiO2 layer. The result, again, is that the local modal gain in the active layer under the window region 135 is higher than the local modal gain in the active layer underthe SiO 2 layer 130. This, combined with the relatively higher effective refractive index in the transverse direction inside the ridge stripe region compared to that outside the ridge stripe region, provides an effective refractive index step (Dn). This results in a structure which has, built in, a waveguide mechanism of a real refractive index guide. Therefore, the design of the second embodiment provides a low threshold current laser diode which can operate with a fundamental transverse mode. Figure 6 shows in graph form the emitted light versus drive current characteristics of a laser diode in accordance with the second embodiment. The laser diode is driven with a cw current. The threshold current is found to be 32.5 mA. Referring next to Figures 7A-7E, a summary of the key fabrication steps is shown for an exemplary device of a semiconductor laser diode in accordance with the second embodiment. Referring first to Figure 7A and 7B, the formation of the first and second cladding layers 105 and 1 10 on an n-type GaN substrate 100, together with the three-pair multiple quantum well active layer 1 15 are the same as for the first embodiment. Thereafter, the third and fourth cladding layers 120 and 125 are formed and then partially removed -- typically by etching -- to create a ridge structure 500. As before, in an exemplary embodiment the various layers are formed successively by either the MOCVD or the MBE method. Then, as shown in Figure 7C-7E, a silicon dioxide layer 130 is formed overthe fifth and third cladding layers 125 and 120, respectively, typically by the CVD method, after which a window region 135 is formed as with the first embodiment. Electrodes 140 and 145 are then evaporated or otherwise bonded to the structure. Referring next to Figure 8, a third embodiment of the present invention may be better appreciated. The third embodiment provides slightly different mole fractions to permit the emission of blue light, but is otherwise similar to the first embodiment. Thus, an n-type GaN substrate 100 continues to be used, together with an n-type GaN first cladding layer 105. However, the second cladding layer 810 is typically of n-type ln 0 15 Ga 070 AI 0 15 N material on the order of 1.5 μm thick, while the three-pair quantum well active layer 815 typically includes three barrier layers of ln 0 15 Ga 084 AI O 01 N material together with four barrier layers of ln 0 16 Ga 0 80 AI 004 N material. The third cladding layer 820 is typically a p-type I n 0 15 Ga 070 Al 0 15 N material, while the fourth cladding layer 125 is, like the first embodiment, a p-type GaN material. The thicknesses of each layer are substantially the same as for the first embodiment. A Si0 2 layer 130, window region 135, and first and second electrodes 140 and 145 complete the structure. In order to emit blue light in a wavelength range of 400 nm from the active layer 24, the mole fractions of InN, GaN, and AIN within the well layer 815 are set to be 0.15, 0.84, and 0.15, respectively. In order to match the lattice constants of the constituent layers to avoid generation of phase separation-induced defects, the GaN mole fraction, x, and the AIN mole fraction, y, of each of the layers is set to satisfy the condition x + 1.2y nearly equals a constant value on the order of 0.85+0.1 ; as before, most embodiments will be in the range 0.85+0.05. Although the third embodiment emits blue light whereas the first embodiment emits ultraviolet light, the band gap energies of cladding layers continue to be set higher than the band gap energy of the three pairs of the multiple quantum well active layer 815. As before, the permits carrier confinement and recombination in the active layer 815. Also as with the first embodiment, the refractive index of the second and third cladding layers is, by design, smaller than that of the active layer, causing the optical field to be confined in the transverse direction. Likewise, the strong current injection under the window region 135 yields comparatively higher local modal gain in the active layer relative to the portion of the active layer underthe Si02 layer 130, again resulting in a guided waveguide mechanism which leads to a lasing oscillation. Figure 9 shows a plot of the emitted light versus drive current characteristics of the laser diode in accordance with the third embodiment. The laser diode is driven with a pulsed current with a duty cycle of 1 %. The threshold current density is found to be 5.0 kA cm 2 . Figures 10A-10D show a series of the fabrication steps of a semiconductor laser diode in one example of the third embodiment It will be appreciated that the fabrication steps are the same as those described in connection with Figures 4A-4D, and therefore are not further described Referring next to Figure 1 1 , a fourth embodiment of the present invention may be better appreciated The fourth embodiment, like the third embodiment, is designed to emit blue light and therefore has the same mole fractions as the third embodiment. However, like the second embodiment, the fourth embodiment is configured to provide a ridge structure to serve as a waveguide. Because the mole fractions are similar to those of Figure 8, similar elements will be described with the reference numerals used in Figure 8. Continuing to refer to Figure 1 1 , the structure of the fourth embodiment can be seen to have a GaN substrate 100 on which is a formed a first cladding layer 105 followed by a second cladding layer 810. A three-pair multiple quantum well active layer 815 is formed thereabove, followed by a third cladding layer 820. A fourth cladding layer 125, silicon dioxide layer 130, windows 135 and electrodes 140 and 145 are all formed as before. The materials, including the mole fractions of InN, GaN, and AIN, remain as shown for Figure 8, or 0.15, 0.84, and 0.01 , respectively. Likewise the GaN mole fraction, x, and the AIN mole fraction, y, of the layers is set to satisfy the condition x + 1.2y is equal to or nearly equal to a constant value on the order of 0.85+0.1 , as with the prior embodiments. The band gap energy, refractive index and modal gain for current injection are all substantially as discussed in connection with the third embodiment and are not further discussed. Figure 12 plots drive current versus emitted light of a laser diode constructed in accordance with the fourth embodiment. The laser diode is driven with a cw current. The threshold current is found to be 28.5 mA. Figure 13 shows a summary of the fabrication steps of a semiconductor laser diode in accordance with the fourth embodiment. The steps are essentially identical to those discussed in connection with Figures 7A-7E and are not further discussed. Referring next to Figure 14, the selection of the GaN mole fraction, x, and the AIN mole fraction, y, and the relationship therebetween for the constituent InGaAIN layers, may be better understood. In particular, the relative mole fractions are required to satisfy, approximately, the relationship 0 < x + y <1 , 1 <= x/0.80 + y/0.89. Figure 14 shows the boundary of phase separation region plotted against various growth temperatures. The lines in Figure 14 show the boundary between the compositionally unstable (phase separation) region and stable region with respect to various temperatures. The region surrounded with the InN-AIN line and the boundary line shows the phase separation content region. It has been discovered that the ternary alloys InAIN and InGaN have a large phase separation region due to the large lattice mismatch between InN and AIN, and between InN and GaN. On the other hand, it is found that the ternary alloy GaAIN has no phase separation region for crystal growth at temperatures around 1000 °C, due to the small lattice mismatch between AIN and GaN. It has therefore been discovered that an InGaAIN material system can be provided in which the usual crystal growth temperature is in the approximate range of around 500 °C to around 1000 °C. Likewise, it has been discovered that phase separation of the In content, Ga content, and Al content of InGaAIN does not occur significantly at processing temperatures between on the order of 500 °C and on the order of 1000 °C. Finally, by combining the two, the content choice region of Ga content and Al content in InGaAIN to avoid phase separation at a crystal growth temperature below around 1000 °C is found to be the shadow region in Figure 15, with the line separating the two regions being approximately defined by the relationship x/0.80 + y/0.89 =1. Therefore, for each of the four structural embodiments disclosed hereinabove, the phase separation phenomena can be avoided in an InGaAIN material system by operating at a crystal growth temperature between on the order of 500 °C and around 1000 °C, when the Ga mole fraction, x, and the AIN mole fraction, y, of the all constituent layers of the laser diodes are made to satisfy approximately the relationship of 0 < x + y <1 , 1 <= x/0.80 + y/0.89. The result is the substantially uniform distribution of In atoms, Ga atoms, and Al atoms in each constituent layer according to the atomic mole fraction. Figure 16 shows the content choice line of Ga content, x, and Al content, y, in an InGaAIN system to avoid phase separation phenomenon at growth temperatures below around 1000 °C. The line in Figure 16 shows the exemplary line of x + 1 .2y = 1 . Therefore, by ensuring that the Ga content and Al content of the constituent InGaAIN layers of a laser diode formed on a GaN substrate have a relationship of x + 1 .2y nearly equal to 1 and 0 < x + y <1 , 1 <= x/0.80 + y/0.89, a laser diode on a GaN substrate with low defect density and no or very little phase separation can be obtained. In addition, other semiconductor structures can be fabricated with the materials system discussed above. Group-Ill nitride materials, especially GaN and AIN, are promising for use in electronic devices which can operate under high-power and high-temperature conditions - for example, microwave power transistors. This results, in part, from their wide band gap (3.5 eV for GaN and 6.2 eV for AIN), high breakdown electronic field, and high saturation velocity. By comparison, the band gaps of AIAs, GaAs, and Si are 2.16 eV, 1 .42 eV, and 1 .12 eV, respectively. This has led to significant research in the use of AIGaN/GaN materials for such field effect transistors (FETs). However, as noted previously hereinabove, the different lattice constants of AIGaN and GaN cause the generate of significant defects, limiting the mobility of electrons in the resultant structure and the utility of such materials systems for FET use. The present invention substantially overcomes these limitations, in that the InGaAIN/GaN material of the present invention has a lattice constant equal to GaN. As discussed hereinabove, a quaternary materials system of In^. y Ga x ALN, where the GaN mole fraction (x) and AIN mole fraction (y) satisfy the relationships 0 < x + y < 1 , 1 <= x/0.8 + y/0.89 and x + 1.2y = 1+0.1 not only has a band gap greater than 3.1 eV, but also has a lattice constant substantially equal to GaN. This permits fabrication of semiconductor structures such as FETs which have substantially uniform atomic content distribution in the various layers. Therefore, by using an InGaAIN/GaN material system in accordance with the present invention, whose GaN mole fraction, x and AIN mole fraction, y satisfy the above relationships, high-power and high-temperature transistors with low defect density can be realized. Referring to Figure 17A, there is shown therein an exemplary embodiment of a heterojunction field effect transistor(HFET) using InGaAIN/GaN material in accordance with the present invention. On a GaN substrate 520, a 0.5 μm i-GaN layer 525 is formed, followed by a thin, approximately 10 nm GaN conducting channel layer 530 and a 10 nm InGaAIN layer 535. Source and drain electrodes 540A-B, and gate electrode 545 are formed in a conventional manner. In the structure, the GaN mole fraction, x, and AIN mole fraction, y, of the InGaAIN layer are set to be 0.64 and 0.3, respectively. In this case, the value of x and y satisfy the relationship of 0 < x + y < 1 , 1<= x/0.8 + y/0.89, and x + 1.2y = 1 +0.1. This results in an InGaAIN layer substantially without phase separation and with a lattice constant equal to GaN, In turn, this permits high electron velocities to be achieved because the two dimensional electron gas formed in the heterointerface of InGaAIN and GaN layer is not scattered by any fluctuation in atomic content of the InGaAIN layer (such as would be caused in the presence of defects). Moreover, the band gap of the InGaAIN is larger than 4 eV so that reliable high-temperature operation can be achieved by using the structure shown in Figure 17A. Similarly, Figure 17B shows an embodiment of a heterojunction bipolar transistor(HBT) in accordance with the present invention. On the GaN substrate 550, a 400 nm thick n-type InGaAIN collector layer 555 is formed, followed by a 50 nm thick p-type GaN base layer 560, and a 300 nm thick emitter layer 565. Base electrode 570, collector electrode 575 and emitter electrode 580 are formed conventionally. As with Figure 17A, forthe exemplary embodiment of Figure 17B the GaN and AIN mole fractions x and y of the InGaAIN layer are set to be 0.64 and 0.3, respectively, and x and y are required to satisfy the same relationships as discussed above. As with Figure 17A, an InGaAIN layer without significant phase separation and with a lattice constant equal to GaN is realized, resulting in a very high quality heterojunction of InGaAIN/GaN. In addition, the band gap of the InGaAIN emitter layer (4.2 eV) is largerthan that of GaN base layer (3.5eV) so that holes generated in the p-type base layer are well confined in that base layer. This results because ofthe largervalence band discontinuity between GaN and InGaAIN than would occur in a GaN homojunction bipolar transistor. This has the benefit of obtaining a large current amplification of collector current relative to base current. Moreover, as mentioned above, the bandgap ofthe InGaAIN and the GaN layer is large so that the transistor can be used reliably in high-temperature applications. Referring next to Figure 8, there is shown therein an implementation of the present invention as a phototransistor. In this regard, GaN and AIGaN are attractive materials for photo detectors in ultraviolet(UV) range, since both GaN and AIN have a wide band gap (3.5 eV for GaN which corresponds to the light wavelength of 200 nm, 6.2 eV for AIN which corresponds to the light wavelenght of 350 nm). Due to the direct band gap and the availability of AIGaN in the entire AIN alloy composition range, AIGaN/GaN based UV photo detectors have the advantage of high quantum efficiency, as well as tunability of high cut-off wavelength. However, the lattice constant of AIGaN is sufficiently different from GaN that defects tend to be formed, which leads increased leakage current. In^. y Ga x ALN quaternary material whose GaN mole fraction, x and AIN mole fraction, y satisfy the relationship of 0 < x + y < 1 , 1 <= x/0.8 + y/0.89, offers not only a band gap largerthan 3.1 eV, but also can be fabricated in layers with equal atomic content distribution, so that InGaAIN material also can be used for UV photo detector applications. Moreover, the ln.,_ x . y Ga x Al y N quaternary material whose GaN mole fraction, x and AIN mole fraction, y satisfy the relationship of x + 1.2y = 1 has a lattice constant equal to GaN. Therefore, by using InGaAIN/GaN material whose GaN mole fraction, x and AIN mole fraction, y satisfy the above relationship, UV photo detectors with low defect density can be realized. In the event that detection of other frequencies is desired, for example blue light, only slight modification is required. As shown in Figure 18, the semiconductor device ofthe present invention can be implemented as a heterojunction phototransistor(HPT) using InGaAIN/GaN material. On the GaN substrate 700, an InGaAIN collector layer 705 is formed on the order of 500 nm thick n-type, followed by the formation of a 200 nm thick p-type GaN base layer 710. Thereafter, an emitter layer 715 on the order of 500 nm thick is formed. On the emitter layer, a ring shaped electrode 720 is formed to permit light to impinge on the base layer. In an exemplary structure, the GaN mole fraction, x and AIN mole fraction, y of the InGaAIN layer are set to be 0.64 and 0.3, respectively. In this case, the value of x and y satisfy the relationship of 0 < x + y < 1 , 1 <= x/0.8 + y/0.89, and x + 1.2y = 1 , so that an InGaAIN layer can be formed with substantially avoids phase separation while having a lattice constant equal to GaN, thus permitting the formation of a high quality heterojunction of InGaAIN/GaN. The band gap of the InGaAIN emitter layer (4.2 eV which corresponds to the light wavelength of 290 nm) is larger than that of GaN base layer (3.5eV which corresponds to the light wavelength of 350 nm). The light impinges on the emitter side. For the embodiment shown, impinging light in the wavelength range between 290 nm and 350 nm is transparent to the emitter layer, so that the light in that range is absorbed in the GaN base layer and generates electron and hole pairs. The holes generated by the optical absorption in the p-type base layer are well confined in the base layer because the valence band discontinuity between GaN and InGaAIN is larger than that for a conventional GaN homojunction photo transistor. This leads to the induction of a larger emitter current, which offers better electronic neutralization in the base region than in the case of the homojunction photo transistor. Therefore, UV photo detectors with high quantum efficiency and high sensitivity, and the resultant high conversion efficiency from input light to collector current, are obtained. In the event that other frequencies are to be detected, the GaN base layer may be replaced with, for example for blue light, InGaN. In addition to the phototransistor embodiment of Figure 18, it is also possible to implement a photodiode in accordance with the present invention. Referring to Figure 19, an n-type substrate 900 is provided, on which is formed an n-type GaN first cladding layer 905. Above the cladding layer is formed an n-type layer 910 of ln.._ x . y Ga x Al y N quaternary material or equivalent, which conforms to the relationships discussed above in connection with Figure 18. An active layer 915 is thereafter formed, and above that is formed a layer 920 of p-type In^. y Ga ^ A N quaternary material. Then, a p-type second cladding layer 925 is formed above the layer 920, and a window 930 is formed therein to expose a portion of the layer 920. The window 930 provides a port by which light can impinge on the layer 920, causing the creation of holes. A pair of electrodes 935 and 940 may be fabricated in a conventional manner, with the electrode 935 typically being a ring electrode around the window 930. It will be appreciated that the band gap of the second cladding layer 925 is preferably larger than the band gap of the layer 920, which is in turn preferably larger than the band gap of the active layer 915; such an approach provides sensitivity to the widest range of wavelengths of light. If the event a narrower range is desired, a material with a lower band gap than the layer 920 may be used for the layer 925. In addition, it is also not necessary to include the layer 925 in all embodiments, as the layers 910, 915 and 920 provide, in at least some instances, an adequate photosensitive pn-junction. Having fully described a preferred embodiment of the invention and various alternatives, those skilled in the art will recognize, given the teachings herein, that numerous alternatives and equivalents exist which do not depart from the invention. It is therefore intended that the invention not be limited by the foregoing description, but only by the appended claims.

Claims: We claim: 1. A semiconductor structure comprising: a first cladding layer of InGaAIN material having a first conduction type, an InGaAIN active layer, and a second cladding layer of InGaAIN material having a conduction type opposite the first conduction type, the mole fractions of the constituent elements of each layer being selected to minimize phase separation. 2. A semiconductor structure comprising: a first cladding layer of InGaAIN material having a first conduction type, an InGaAIN active layer, and a second cladding layer of InGaAIN material having a conduction type opposite the first conduction type, the crystal growth temperature and the mole fractions of the constituent elements of each layer being selected to minimize phase separation. 3. A light emitting device according to claim 1 , wherein the GaN mole fraction, x, and the AIN, y, of all the constituent layers satisfy the condition that x + 1.2y nearly equals to a constant value. 4. A light emitting device according to claim 1 , wherein the GaN mole fraction, x, and the AIN, y, of all the constituent layers satisfy the condition that x + 1.2y nearly equals to 1. 5. A light emitting device comprising: a first conduction type of an InGaAIN first cladding layer without phase separation, an InGaAIN active layer without phase separation, and a second conduction type of InGaAIN second cladding layer without phase separation, said InGaAIN second cladding layer having a ridge structure, all successively formed one upon each other. 6. A light emitting device according to claim 4, wherein the GaN mole fraction, x, and the AIN, y, of all the constituent layers satisfy the condition that x + 1.2y nearly equals constant value. 7. A light emitting device according to claim 4, wherein the GaN mole fraction, x, and the AIN, y, of all the constituent layers satisfy the condition that x + 1.2y nearly equals 1. 8. A light emitting device comprising: a certain conduction type of a GaN first cladding layer, an ln 1 . x1 . y1 Ga x1 Al y1 N second cladding layer of said certain conduction type, an In^.^Ga^AI^N active layer, an opposite conduction type of an ln 1 . x1 _ y1 Ga x1 Al y1 N third cladding layer, an opposite conduction type of GaN fourth cladding layer, all successively formed one upon each other, wherein x1 , x2 define the gallium mole fraction, y1 and y2 define the aluminum mole fraction, and x1 , y1 , x2, and y2 have a relationship of 0 < x1 + y1 <1 , 0 < x2 + y2 <1 , 1<= x1/0.80 + y1/0.89, 1 <= x2/0.80 + y2/0.89, and Eg lnN (1 -x1-y1 ) + Eg GaN x1 + Eg AIN y1 > Eg lnN (1-x2-y2) + Eg GaN x2 + Eg AIN y2, where Eg lnN , Eg GaN , and Eg A , N are the bandgap energy of InN, GaN, and AIN, respectively. 9. A light emitting device according to claim 7, wherein said active layer is an InGaAIN single or multiple quantum well active layer whose GaN mole fraction, xw, and AIN mole fraction, yw of all the constituent layers of said active layer satisfy the relationship of 0 < xw + yw <1 and 1 <= x/0.80 + y/0.89. 10. A light emitting device according to claim 7, wherein the condition of xs + 1.2ys nearly equals to a constant value is satisfied, wherein xs and ys are the GaN mole fraction and the AIN mole fraction, respectively in each constituent InGaAIN layers. 11. A light emitting device according to claim 7, wherein the relationship of xs + 1.2ys nearly equals 1 is satisfied, where xs and ys are the GaN mole fraction and the AIN mole fraction, respectively in each constituent InGaAIN layers. 12. A light emitting device comprising: a certain conduction type of an ln 1 . x1 . y1 Ga x1 Al y1 N first cladding layer of said certain conduction type, an In^.^Ga^AI^N active layer, an opposite conduction type of an second cladding layer has a ridge strict, all successively formed one upon each other, wherein x1 , x2, and x3 define the gallium mole fraction, y1 , y2, and y3 define the aluminum mole fraction, and x1 , y1 , x2, y2, x3, and y3 have a relationship of 0 < x1 + y1 <1 , 0 < x2 + y2 <1 , 0 < x3 + y3 <1 , 1 <= x1/0.80 + y1/0.89, 1 <= x2/0.80 + y2/0.89, 1 <= x3/0.80 + y3/0.89, Eg lnN (1-x1-y1 ) + Eg GaN x1 + Eg AIN y1 > Eg lnN (1-x2-y2) + Eg GaN x2 + Eg AIN y2, and Eg lnN (1-x3-y3) + Eg GaN x3 + Eg AlN y3 > Eg lnN (1-x2-y2) + Eg GaN x2 + Eg AlN y2, where Eg, nN , Eg GaN , and Eg A , N are the bandgap energy of InN, GaN, and AIN, respectively. 13. A light emitting device according to claim 11 , wherein said active layer is an InGaAIN single or multiple quantum well active layer whose GaN mole fraction, xw, and AIN mole fraction, yw of all the constituent layers satisfy the relationship of 0 < xw + yw <1 and 1 <= x/0.80 + y/0.89. 14. A light emitting device according to claim 11 , wherein the condition of xs + 1.2ys nearly equals to a constant value is satisfied, where xs and ys are the GaN mole fraction and the AIN mole fraction, respectively in each constituent InGaAIN layers. 15. A light emitting device according to claim 11 , wherein the relationship of xs + 1.2ys nearly equals 1 is satisfied, wherein xs and ys are the GaN mole fraction and the AIN mole fraction, respectively in each constituent InGaAIN layers. 16. A photo detector device comprising: a certain conduction type of an InGaAIN collector layer without phase separation, a opposite conduction type of InGaAIN base layer without phase separation, a conduction type of said certain conduction type of InGaAIN emitter layer without phase separation, all successively formed one upon each other, wherein the bandgap of said InGaAIN base layer is smaller than the other InGaAIN layers. 17. A photo detector device according to claim 16, wherein the GaN mole fraction, x, and the AIN, y, of the said constituent InGaAIN layers satisfy the relationship of 0 < x + y <1 , 1 <= x1/0.80 + y1/0.89. 18. A photo detector device according to claim 17, wherein the GaN mole fraction, x, and the AIN, y, of all the constituent layers satisfy the condition that x + 1.2y nearly equals to a constant value. 19. A photo detector device according to claim 17, wherein the GaN mole fraction, x, and the AIN, y, of all the constituent layers satisfy the condition that x + 1.2y nearly equals to 1.

Inventors: HARRIS JAMES S JR, TAKAAKI BABA, TORU TAKAYAMA, HARRIS, JAMES, S., JR., TAKAAKI, BABA, TORU, TAKAYAMA
Applicants: MATSUSHITA ELECTRIC CORP, MATSUSHITA ELECTRIC CORPORATION
Application year: 2000
Main classification: H01S5
IPC codes: H01L29, H01L33, H01S5