H. Morkoç

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.

Valence‐band discontinuities of wurtzite GaN, AlN, and InN heterojunctions measured by x‐ray photoemission spectroscopy

The valence‐band discontinuities at various wurtzite GaN, AlN, and InN heterojunctions were measured by means of x‐ray photoemission spectroscopy. A significant forward–backward asymmetry was observed in the InN/GaN–GaN/InN and InN/AlN–AlN/InN heterojunctions. The asymmetry was understood as a piezoelectric strain effect. We report the valence band discontinuities for InN/GaN=1.05±0.25 eV, GaN/AlN=0.70±0.24 eV, and InN/AlN=1.81±0.20 eV, all in the standard type I lineup. These values obey transitivity to within the experimental accuracy. Tables of photoemission core level binding energies are reported for wurtzite GaN, AlN, and InN.

Gallium arsenide and other compound semiconductors on silicon

The physics of the growth mechanisms, characterization of epitaxial structures and device properties of GaAs and other compound semiconductors on Si are reviewed in this paper. The nontrivial problems associated with the heteroepitaxial growth schemes and methods that are generally applied in the growth of lattice mismatched and polar on nonpolar material systems are described in detail. The properties of devices fabricated in GaAs and other compound semiconductors grown on Si substrates are discussed in comparison with those grown on GaAs substrates. The advantages of GaAs and other compound semiconductors on Si, namely, the low cost, superior mechanical strength, and thermal conductivity, increased wafer area, and the possibility of monolithic integration of electronic and optical devices are also discussed.

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.

Resonant cavity-enhanced (RCE) photodetectors

The photosensitivity characteristics of resonant cavity-enhanced (RCE) photodetectors are investigated. The photodetectors were formed by integrating the active absorption region into a resonant cavity composed of top and bottom (buried) mirrors. A general expression for quantum efficiency for RCE photodetectors was derived taking the external losses into account. Drastic enhancement in quantum efficiency is demonstrated at resonant wavelengths for a high quality factor Q cavity with a very thin absorption layer. An improvement by a factor of four in the bandwidth-efficiency product for RCE p-i-n detectors is predicted. Molecular beam epitaxy grown RCE-heterojunction phototransistors (RCE-HPT) were fabricated and measured demonstrating good agreement between experiment and theory. >

Similarities in the Bandedge and Deep-Centre Photoluminescence Mechanisms of ZnO and GaN

Abstract Several of the optical transitions in ZnO and GaN appear to derive from a similar origin and have considerable overlap in the energy regions where they occur. In particular the donor-acceptor pair transitions and the well known “yellow band” in GaN and the analogous “green band” in ZnO show remarkable similarities. Because of these similarities it is likely that the respective transitions in the two materials are defect related and share common mechanisms.

An investigation of the properties of cubic GaN grown on GaAs by plasma‐assisted molecular‐beam epitaxy

We present the first comprehensive investigation of the bulk properties, both optical and structural, of cubic GaN as grown by plasma‐assisted molecular‐beam epitaxy on vicinal (100) GaAs substrates. X‐ray measurements determined the crystal structure of GaN/GaAs to be cubic with a lattice constant of 4.5 A. High resolution transmission electron microscopy revealed a high density of planar defects propagating along the GaN {111} planes. The majority of the defects originated from disordered regions at the GaN/GaAs interface. The optical properties of the films were investigated by cathodoluminescence which revealed a broad midgap peak as well as several sharp emission peaks just below the expected band gap. The data imply that the room temperature band gap of cubic GaN is approximately 3.45 eV.

Microstructure of Ti/Al and Ti/Al/Ni/Au Ohmic contacts for n-GaN

Transmission electron microscopy has been applied to characterize the structure of Ti/Al and Ti/Al/Ni/Au Ohmic contacts on n‐type GaN (∼1017 cm−3) epitaxial layers. The metals were deposited either by conventional electron‐beam or thermal evaporation techniques, and then thermally annealed at 900 °C for 30 s in a N2 atmosphere. Before metal deposition, the GaN surface was treated by reactive ion etching. A thin polycrystalline cubic TiN layer epitaxially matched to the (0001) GaN surface was detected at the interface with the GaN substrate. This layer was studied in detail by electron diffraction and high resolution electron microscopy. The orientation relationship between the cubic TiN and the GaN was found to be: {111}TiN//{00.1}GaN, [110]TiN//[11.0]GaN, [112]TiN//[10.0]GaN. The formation of this cubic TiN layer results in an excess of N vacancies in the GaN close to the interface which is considered to be the reason for the low resistance of the contact.

Material properties of high‐quality GaAs epitaxial layers grown on Si substrates

We report an investigation of the materials properties of GaAs on Si epitaxial layers. By using properly oriented substrates, we have found that a substantial reduction in the density of threading dislocations can be achieved. In the presence of steps, dislocations with their Burgers vectors in the (100) substrate plane are preferentially generated, which are more effective in accommodating lattice mismatch and do not thread into the epitaxial layer. We have also found that the density of threading dislocations can be reduced significantly by the use of GaAs/InGaAs pseudomorphic superlattices. Using these techniques, dislocation densities of as low as 103–104 cm−2 have been achieved in 2‐μm‐thick GaAs on Si epitaxial layers. In growth on nominal (100) orientations, where it is known that single atomic steps dominate, we have found no evidence of antiphase domains by transmission electron microscopy or chemical etching. This result suggests that it may not be energetically favorable for antiphase domains t...

Optical investigation of highly strained InGaAs‐GaAs multiple quantum wells

Low‐temperature optical transmission spectra of several InxGa1−xAs/GaAs strained multiple quantum wells (MQWs) with different well widths and In mole fractions have been measured. The excitonic transitions up to 3C‐3H are observed. The notation nc‐mH(L) is used to indicate the transitions related to the nth conduction and mth valence heavy (light) hole subbands. Steplike structures corresponding to band‐to‐band transitions are also observed, which are identified as 1C‐1L transitions. The calculated transition energies, taking into account both the strain and the quantum well effects, are in good agreement with the measured values. In these calculations the lattice mismatch between the GaAs buffer and the InGaAs/GaAs MQW is taken into account and the valence‐band offset Qv is chosen as an adjustable parameter. By fitting the experimental results to our calculations, we conclude that the light holes are in GaAs barrier region (type II MQW) and the valence‐band offset Qv is determined to be 0.30. A possible ...