M. E. Lin

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

In the past several years, research in each of the wide‐band‐gap semiconductors, SiC, GaN, and 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 GaN should enable them to operate at 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 demonstrated operation in the green, blue, or ultraviolet (UV) spectra. Blue SiC light‐emitting diodes (LEDs) have been on the market for 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 ZnSe 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 to be aggressively pursued to further 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 development can rapidly be brought to maturity.

Low resistance ohmic contacts on wide band‐gap GaN

We report a new metallization process for achieving low resistance ohmic contacts to molecular beam epitaxy grown n‐GaN (∼1017 cm−3) using an Al/Ti bilayer metallization scheme. Four different thin‐film contact metallizations were compared during the investigation, including Au, Al, Ti/Au, and Ti/Al layers. The metals were first deposited via conventional electron‐beam evaporation onto the GaN substrate, and then thermally annealed in a temperature range from 500 to 900 °C in a N2 ambient using rapid thermal annealing techniques. The lowest value for the specific contact resistivity of 8×10−6 Ω cm2, was obtained using Ti/Al metallization with anneals of 900 °C for 30 s. X‐ray diffraction and Auger electron spectroscopy depth profile were employed to investigate the metallurgy of contact formation.

REACTIVE ION ETCHING OF GAN USING BCL3

Reactive ion etching with SiCl4 and BCl3 of high quality GaN films grown by plasma enhanced molecular beam epitaxy is reported. Factors such as gas chemistry, flow rate, and microwave power affecting the etching rate are discussed. The etch rate has been found to be larger with BCl3 than with SiCl4 plasma. An etch rate of 8.5 A/s was obtained with the BCl3 plasma for a plasma power of 200 W, pressure of 10 mTorr, and flow rate of 40 sccm. Auger electron spectroscopy (AES) was used to investigate the surface of GaN films after etching. Oxygen contamination has been detected from the AES profiles of etched GaN samples.

GaN grown on hydrogen plasma cleaned 6H-SiC substrates

We report epitaxial GaN layers grown on 6H‐SiC (0001) substrates. A two stage substrate preparation procedure is described which effectively removes oxygen from the SiC substrate surface without the need of elaborate high temperature processing. In the first step, dangling Si bonds on the substrate surface are hydrogen passivated using a HF dip before introduction into vacuum. Second, the substrate is treated with a hydrogen plasma reducing the amount of oxygen‐carbon bonding to below the x‐ray photoemission detection limit. Upon heating in the molecular beam epitaxy (MBE) growth chamber, the SiC substrates are observed to have a sharp (1×1) reconstruction with Kikuchi lines readily visible. GaN epilayers deposited on AlN buffer layers by plasma enhanced MBE show sharp x‐ray diffraction and photoluminescence peaks.

A comparative study of GaN epilayers grown on sapphire and SiC substrates by plasma‐assisted molecular‐beam epitaxy

We report structural, electrical, and optical data for GaN samples grown on both 6H‐SiC and sapphire substrates. A two‐stage substrate preparation procedure was employed for removing oxygen from 6H‐SiC and c‐plane sapphire substrates without the need for elaborate high‐temperature thermal degassing. Both sapphire and SiC substrates were treated with hydrogen plasma to reduce the surface contamination as evidenced by the observation of sharp (1×1) reconstruction RHEED (reflected high‐energy electrons diffraction) patterns. Thin AlN buffer layers were employed and the crystalline quality of GaN films was studied by temperature‐dependent Hall measurements, photoluminescence, and x‐ray diffraction. Layers with room‐temperature mobilities as high as 580 cm2/V s on SiC substrates were obtained.

p‐type zinc‐blende GaN on GaAs substrates

We report p‐type cubic GaN. The Mg‐doped layers were grown on vicinal (100) GaAs substrates by plasma‐enhanced molecular beam epitaxy. Thermally sublimed Mg was, with N2 carrier gas, fed into an electron‐cyclotron resonance source. p‐type zinc‐blende‐structure GaN films were achieved with hole mobilities as high as 39 cm2/V s at room temperature. The cubic nature of the films were confirmed by x‐ray diffractometry. The depth profile of Mg was investigated by secondary ions mass spectroscopy.

Nonalloyed ohmic contacts on GaN using InN/GaN short‐period superlattices

It is well known that ohmic contacts on GaN, a highly promising material for electronic and optoelectronic devices with a wide band gap of about 3.4 eV, constitute a major obstacle to further development of devices based on this material. We demonstrated a novel scheme of nonalloyed ohmic contacts on GaN using a short‐period superlattice (SPS), composed of GaN and narrow band‐gap InN, sandwiched between the GaN channel and an InN cap layer. Comparison with a similar layer without the SPS structure indicates that quantum tunneling through the SPS conduction band effectively reduces the potential barrier formed by the InN/GaN heterostructure leading to low contact resistivities. From the transmission‐line‐method measurements, specific contact resistances as low as 6×10−5 Ω cm2 with GaN doped at about 5×1018 cm−3 have been obtained without any post‐annealing. Theoretical estimation based on the SPS tunneling model is consistent with the experiment.

Characterization of Group III-nitride semiconductors by high-resolution electron microscopy

Abstract High-resolution electron microscopy has been used to characterize the microstructure of thin films of GaN, AlN and InN, as grown by plasma-enhanced molecular beam epitaxy. Zincblende and wurtzite polytypes were preferentially nucleated using (001) GaAs and (0001) 6H SiC substrates, respectively. Stacking faults and microtwins along 111 planes dominated the zincblende films, whereas stacking faults along 0002 planes and threading defects originating at the substrate surface were most prevalent in the wurtzite phase. Improved crystal quality was achieved by growing the films on suitable buffer layers.

Growth and characterization of GaN on c-plane (0001) sapphire substrates by plasma-enhanced molecular beam epitaxy

GaN films were grown on the c‐plane of sapphire substrates by plasma‐enhanced molecular beam epitaxy equipped with an electron‐cyclotron resonance (ECR) source. Sapphire substrates were cleaned by a hydrogen plasma treatment, obviating the need for perilous elevated temperatures. ECR sources are plagued with high energetic ions, particularly at high microwave power levels, where they cause damage in the growing films. To circumvent this problem, we systematically optimized the growth conditions and other pertinent parameters for optimum layer quality. Among the parameters optimized were the magnetic field strength, microwave power, nitrogen over‐pressure, and growth temperature. The quality of the GaN layers were evaluated by electrical and structural measurements as well as observing the surface morphology.

Thermal stability of GaN investigated by low-temperature photoluminescence spectroscopy

We have studied the thermal behavior of 1‐μm‐thick GaN films grown by plasma‐enhanced molecular beam epitaxy. Samples were annealed at elevated temperatures in a nitrogen environment and were characterized by low‐temperature photoluminescence (PL). After GaN samples were annealed at up to 700 °C, the free‐exciton transition PL line intensity improved. This PL line intensity degraded when annealing temperatures reached 900 °C. After annealing at 900 °C, GaN samples with inferior crystalline quality exhibited a line at 2.3 eV attributed to point defects and antisite defects.