Large‐band‐gap SiC, III‐V nitride, and II‐VI ZnSe‐based semiconductor device technologies

Large-band-gap SIC, Ill-V nitride, and II-VI ZnSe-based semiconductor device technologies H. MorkoG, S. Strite,a) G. B. Gao, M. E. Lin, B. Sverdlov, and M. Burns University of Illinois, Materials Research Laboratory and Coodinated Science Laboratory 104 South Goodwin Avenue, Urbana, Illinois 61801 (Received 13 January 1993; accepted for publication 17 March 1994) ‘- In the past several years, research in each of the wide-band-gap semiconductors, Sic,. GaN, and a ZnSe, has led to major advances which now make them viable for device applications. The merits of each contender for high-temperature electronics and short-wavelength optical applications are compared. The outstanding thermal and chemical stability of Sic and GaNshould enable them to operate ai high temperatures and in hostile environments, and ‘also make them attractive for high-power operation. The present advanced stage of development of Sic substrates and ’ metal-oxide-semiconductor technology makes Sic the leading contender for high-temperature and high-power applications if ohmic contacts and interface-state densities can be further improved. GaN, despite fundamentally ,-superior electronic properties and better ohmic contact resistances, must overcome the lack of an ideal substrate material and a relatively advanced Sic infrastructure in order to compete in electronics applications. Prototype transistors have been fabricated from both Sic and GaN, and the microwave characteristics and high-temperature performance of Sic transistors. have been studied. For optical emitters and detectors, ZnSe, Sic, and GaN all have c. demonstrated operation in the green, blue, or ultraviolet (UV) spectra. Blue Sic light-emitting ” ~~ diodes (LEDs) have been on the market fo;-several years, joined recently by UV and blue GaN-based LEDs. These products should find wide use in full color display and other technologies. Promising prototype UV photodetectors have been fabricated from both Sic and GaN. In laser development, ZnSe leads the way with more sophisticated designs -having further improved performance being rapidly demonstrated. If the low damage threshold of Z&e continues to limit practical laser applications, GaN appears poised to become the semiconductor of choice for short-wavelength lasers in optical memory and other applications. For further development of these materials to be realized, doping’densities (especially p type) and-ohmic contact technologies have to be improved. Economies of scale need to be realized through the development of larger Sic substrates. Improved substrate materials, ideally GaN itself, need tb be aggressively pursued to furthe; develop the GaN-based material system-and enable the fabrication of lasers. ZnSe material quality is already outstanding and now researchers must focus their attention on addressing the short lifetimes of ZnSe-based lasers to determine whether the material is sufficiently durable for practical laser applications. The problems related to these three wide-band-gap semiconductor systems have moved away from materials science toward the device arena, where their technological developmtint can rapidly be brought to maturity. TABLE OF CONTENTS I. Introduction:. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1364 II. Sic....................................... 1364 J. Sic bipolar transistors. . . . . . . . . . . . . . . . . . . . . 1374 K. Sic light emitting diodes. . . . . . . . . . . . . , . . . . . 1375 L. Sic photodiodes. . . . . . . . . . . . . . . . . . . . . . . . . . 1376 M. Sic ‘rectifiers. . . . . . . . . . . . . . . . . . . . . . . . . . . .’ . 1377 A. B. C. D. E. F. G. H. I. Sic for high power, high temperature N. Remaining issues in Sic and summary ........ 1377 electronics. .............................. 136-q III. GaN based III-V nitride semidonductors. ....... 1379 A. B. C. D.; Polytypisti in SIC. ....................... 1365 Sic substrate crystal growth. ............... 1367 Sic thin film epitaxy. .. : .................. 1367 Dopant considerations. .......... - ......... 1368 Oxidation of Sic. ......................... 1368 Sic etching. ............ :. ............... 1369 Ohmic contacts to Sic. .................... 1370 Sic field effect transistors. ................. 1371 1. Sic power MOSFETs. .................. 1371 2. Sic JFETs ............................ 1372. 3. Sic MESFETs ........................ 1373 E. F. G. H. 1. J. ‘IPresent address: BM Research Division, Ziirich Research Laboratory, Siumerstrasse 4, CH:8803 Riischlikon, Switzerland. K. L. Fundamental properties of G’aN, AlN, and InN 1380 Nitride crystal growth. .. .- ................. 1380 Substrates for nitride epitaxy. ............... -1381 Buffer layers for nitride heteroepitaxy on sapphire. ... .I ........................... 1382 Polytypism in the III-V nitrides. .............. 1382 Electrical properties of undoped nitride thin films ................................... 1382 Properties of doped GaN. .................. 1382 Thermal stability and etching of.GaN. ........ 1384 Ohmic contacts to GaN: ................... 1384 Properties of nitride alloys. ......... t ....... 1385 GaN based LEDs. ..................... . .. F 1386 GaN FETs. .............................. 1388 J. Appl. Phys. 76 (3), 1 August 1994 0021-8979/94/76(3)/i363/36/$6.00 0 1994 American institute of Physics 1383 Downloaded 24 Jun 2010 to 159.226.35.205. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp Administrator 高亮 Administrator 高亮 Administrator 下划线 M.TowardsaGaNlaser.. ..... - ..;. .... /. ..... 1389: N. Negative electron affinity devices. ........... 1389 IV ZnSe based II-VI semiconductor lasers .......... 1389 A. ZnSe crystal growth ....................... 1390 B. Doping ................................. 1390 C. Ohmic contacts ........................... 1390 D. Laser structures. .......................... 1391 E. Outlook for ZnSe as a laser material. ........ 1394 . Summary and discussion. ..................... 1394 I. INTRODUCTION The recent surge of activity in wide-band-gap semicon- ductors has arisen from the need for electronic devices ca- pable of operation at high power levels, high temperatures, and caustic environments, and separately, a need for optical materials, especially emitters, which are active in the blue and ultraviolet (UV) wavelengths. Electronics based on the existing semiconductor device technologies of Si and GaAs cannot tolerate greatly elevated temperatures or chemically hostile environments due to the uncontrolled generation of intrinsic carriers and their low resistance to caustic chemi- cals. The wide-band-gap semiconductors SIC and GaN, and perhaps sometime in the future, diamond, with their excel- lent thermal conductivities, large breakdown fields, and re- sistance to chemical attack, will be the materials of choice for these applications. In the optical device arena, the ever- increasing need for higher-density optical storage and full color display technologies are driving researchers to develop wide-band-gap semiconductor emitter technologies which are capable of shorter-wavelength operation. Much progress has been made in Sic for high- temperature and high-power devices applications due to the availability of high-quality Sic substrates, advances in chemical-vapor-deposition (CVD) growth of epitaxial struc- tures, and the ability to easily dope the material both n and p type. The large Si-C bonding energy makes Sic resistant to chemical attack and radiation, and ensures its stability at high temperatures. In addition, Sic has a large avalanche breakdown field, excellent thermal conductivity, and a high electron saturation velocity, all of which make it ideal for high-power operation. Metal-semiconductor and metal- oxide-semiconductor transistors with outstanding high- temperature performance have already been demonstrated. With recent advancements in the growth of the 3C and 4H polytypes, which have higher electron mobilities than the 6H polytype, Sic is certain to attract continued attention for high-power electronics applications. sensors would reduce complexity and increase reliability.’ Hydraulic systems, a fire hazard in aircraft, and heat radia- tors in satellites could then be greatly reduced in size and number yielding in considerable weight reductions. In the field of optical devices, several trends are pushing research into new materials. The ever-increasing need for denser optical storage media is driving the development of shorter-wavelength semiconductor laser technologies be- cause the diffraction-limited optical storage density increases quadratically as the probe laser wavelength is reduced. To- wards this end, yellow lasers based on InG&AlP heterostruc- hues have been successfully demonstrated; however, this material system is limited to 650 nm. Toward still shorter wavelengths, the recently demonstrated ZnSe-based laser technology is capable of operation in the green and blue wavelengths, and GaN can potentially lase in the UV. Wide-band-gap emitters are also bringing semiconductor technology to full color displays. For the first time, all three primary colors can be generated using semiconductor tech- nology, which promises to allow the reliability, compactness, and other desirable attributes of semiconductors to be ap- plied to this important technological market. In this review, we concentrate on device-oriented re- search and applications of Sic, GaN, and ZnSe. Section II covers Sic and describes recent progress in Sic substrates, epitaxy, processing, and the devices to which these advances have led. Theoretical work detailing the high-frequency and’ high-power performance of Sic and other wide-band-gap semiconductors is also described. Section III examines GaN along with its AlN and InN alloys, covering advances in epitaxial techniques, doping and device technology. ZnSe is reviewed in Sec. IV, with a primary focus on p-type doping with activated nitrogen and the design and fabrication of lasers. Il. SIC Sic was one of the first semiconductors discovered and its large cohesive energy caused some to mistake it for an element. Its high electron saturation velocity, wide band gap, and high thermal conductivity, among other properties, make it highly attractive for high-temperature and high-power ap- plications. Despite its indirect band gap, Sic blue LEDs are commercially available. Below, we describe the material properties, crystal growth, doping, oxidation, etching, ohmic contacts, and device technology of Sic. Industries such as the aerospace, automotive, petroleum, and others have continuously provided the impetus pushing the development of fringe technologies which are tolerant of increasingly high temperatures and hostile environments. Sic and the III-V nitride devices will be capable of improved high-power and -temperature operation due to their large band gaps. GaN may prove superior since it has lower ohmic contact resistances and is predicted to have larger electron saturation velocities. A suitable high-temperature semicon- ductor technology could allow bulky aircraft hydraulics and mechanical control systems to be replaced with heat-tolerant in situ control electronics. On-site electronics, actuators, and A. Sic for high-power, high-temperature electronics An appreciation of the potential of Sic for electronic applications can be gained by examining Table I which com- pares the relevant material properties of Sic and GaN with Si and GaAs, the two most popular semiconductor device technologies, and GaP and diamond, two other contenders for high-temperature applications. Most notable are the large thermal conductivities, breakdown voltages, and saturation velocities of Sic, GaN, and diamond. The device maximum operating temperature parameter is calculated as the tem- perature at which the intrinsic carrier concentration equals 5XlOl’ cme3 and is intended as a rough estimate of the 1364 J. Appl. Phys., Vol. 76, No. 3, 1 August 1994 Morkop et a/. Downloaded 24 Jun 2010 to 159.226.35.205. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp Administrator 下划线 Administrator 打字机 光学存储密度随着 波长的减小二次方增加 TABLE? I. Comparison of important semiconductor properties for high-temperature electronics. Properly Si GaAs GaP 3C Sic (6H SIC) Diamond GaN Band gap (eV) at 300 K 1.1 Maximum operating temperature (K) . -- Melting point (K) Physical stability Good Electron mobility RT, cm’/V s Hole mobility RT, cm2/v s Breakdown voltage Eb , lO’/V/cm Thermal conductivity cr. W/cm Sat. C . elec. drift vel. r&at), 10’ cm/s Dielectric const. K 600? 1690 1400 600 0.3 1.5 1 11.8 1.4 760? 1510 Fair 8500 400 0.4 2.3 1250? 1740 Fair 350 100 0.5 0.8 2 12.8 11;l 2.2 (2.9) 1200 (1580) Sublimes >2100 Excellent 1000 (600) 40 4 5 2 9.7 5.5 3.39 1400(?) Phase change Very good 2200 Good 900 1600 lSO? 10 51 _. 20 1.3 2.7 2.7 5.5 9 band-gap iimitation on device operation. More important for the eventual maximum operating temperature is the physical stability of the material. Figure 1 shows the electron velocity field relationship for Si, GaAs, InP, and SiC.2 The large saturation velocities in Sic are attributable to its larger optical phonon energies. It is clear that Sic can also function well at much higher electric fields than the conventional semiconductors. In Sec. III we discuss theoretical predictions of an even larger electron saturation velocity in GaN. 108 -i 10’ ic...................... .# 4 : 25 106 - ld 104 16 106 10' Electric Field, V/cm FIG. 1. Velocity-field relationships of Si, GaAs, InP, 3C and 6H SIC illustat- ing the superior high-field electron drift velocities and breakdown field of SiC. 3C Sic has a higher saturation velocity than 6H Sic due to reduced phonon scattering. J. Appl. Phys., Vol. 76, No. 3, 1 August 1994 Morkoq eta/. 1365 B. Polytypism in Sic ‘Sic is the most prominent of a family of close-packed materials that exhibit a one-dimensional polymorphism called polytypism. The Sic polytypes are differentiated by the stacking sequence of the tetrahedrally bonded Si-C bilay- ers, such that the individual bond lengths and local atomic environments are nearly identical, while the overall symme- try of the crystal is determined by the stacking periodicity. Ashorthand has been developed to catalogue the literally infinite number’of possible polytype crystal structures. Each Sic bilayer, while maintaining the tetrahedral bonding scheme of the crystal, can be situated in one of three possible positions with respect to the lattice. These are each arbitrarily assigned the notation A, B, or C. Depending on the stacking order, the bonding between Si and C atoms in adjacent bi- layer planes is either of a zinc-blende (cubic) or wurtzite (hexagonal) nature. Zinc-blende bonds are rotated 60” with respect to nearest neighbors while hexagonal bonds are mir- ror images (Fig. 2). Each type of bond provides a slightly altered atomic environment making some lattice sites in- equivalent in polytypes with mixed bonding schemes and reducing the overall crystal symmetry. These effects are im- portant when considering the substitutional impurity incor- poration and electronic transport properties of Sic. If the stacking is ABCABC..., the purely cubic zinc- blende structure, commonly abbreviated as 3C Sic (or beta Sic) is realized (Fig. 3). The number 3 refers to the three bilayer periodicity of the stacking and the letter C denotes the overall cubic symmetry of the crystal. 3C Sic is the only possible cubic polytype. The purely wurtzite ABAB... stack- Downloaded 24 Jun 2010 to 159.226.35.205. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp Administrator 高亮 Zincblende k------c Wurtzite FIG. 2. Zinc-blende and wurtzite bonding between Si and C atoms in adja- cent planes. The three tetrahedral bonds are 60” rotated in the cubic case and mirror images in the hexagonal case. ABCABC ing sequence is denoted as 2H SIC reflecting its two bilayer stacking periodicity and hexagonal symmetry. All of the other polytypes are mixtures of the fundamental zinc-blende and wurtzite bonds. Some common hexagonal polytypes with more complex stacking sequences are 4H and 6H Sic (Figs. 4 and 5). 4H Sic is composed equally of cubic and hexagonal bonds, while 6H SIC is two-thirds cubic. Despite the cubic elements, each has overall hexagonal crystal sym- metry. The family of hexagonal polytypes is collectively re- ferred to as alpha Sic. Rhombehedral structures such as 15R and 21R have also been documented.3 The different polytypes have widely ranging physical properties. 3C Sic has the highest electron mobility and saturation velocity because of reduced phonon scattering re: sulting from the higher symmetry. The band gaps. differ widely among the polytypes ranging from 2.3 eV for 3C Sic to 2.9 eV in 6H Sic to 3.3 eV for 2H Sic. In general, the greater the wurtzite component, the larger the band gap. Among the Sic polytypes, 6H is most easily prepared and best’studjed, while the 3C and 4H polytypes are attracting -- : more attention for their superior electronic properties. The polytypism of SiC makes it nontrivial to grow single-phase material, but it also offers some potential advantages- if crystal-growth methods can be developed sufficiently to capitalize on the possibility of polytype homo/hetero- junctions. Figure 6 shows one such possibility, a 6H/3C Sic interface. Such a junction incorporates the advantages of hec c B A C B A A FIG. 3. Crystal structure of the purely cubic 3C Sic polytype: Each lattice site (k, representing cubic symmetry) is’equivalent. 1366 J. Appl. Phys., Vol. 76, No. 3, 1 August 1994 Morkop et al. V ABCTBCA FIG. 4. Crystal structure of the 4H SIC polytype. Half of the atomic sites are hexagonally bonded (h) while half are cubic (k). erojunction band offsets while maintaining a completely nonpolar, lattice matched, and coherent interface. L. Powell et aL4 have shown that 3C Sic can be grown on well-oriented basal plane (0001) 6H substrates. In this case, the heteroepitaxy is terrace controlled, and the adatoms are free to choose the most energetically favorable stacking se- quence’in the direction perpendicular to the substrate surface. Proper control of the growth conditions gives rise to 3C Sic heteroepitaxy. When the substrate is miscut several degrees away from the basal plane toward (1120) .! steps ‘are prevalent 1.. _. __. .~ . . 7 B C A C B -A B “.- c A’ C B A- ABCABCA FIG. 5. Crystal ~structure of 6H SiCpolytype, the most prevalent polytype. The lattice is two thirds cubic (k, and ka) and one third hexagonal (h t). The two cubic sites are inequivalent and are expected fo have slightly dierent binding energies for substitutional impurities. Downloaded 24 Jun 2010 to 159.226.35.205. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp Administrator 高亮 Administrator 高亮 Administrator 下划线 f 6H Sic (0001) 0 :Si o-c .- Heterointc I *- 3C Sic (111) :rface . FIG. 6. Schematic of a 3U6H SIC polytype heterojunction. When the plane of the interface is normal to the stacking direction, a coherent, lattice- matched heterojunction is possible. on the growth surface. If the step spacing is: less than the adatom surface diffusion length, the growth is step con- trolled, and the