This paper reviews progress in the heteroepitaxial growth of Ill-Nitride semiconductors. The growth of wurtzite and zinc-blende allotropic forms of GaN on various substrates with hexagonal and cubic symmetry respectively were discussed. In particular we addressed the growth on the various faces of sapphire, 6H-SiC and (001) Si. It has been shown that the kinetics of growth by plasma-MBE or ammonia-MBE are different. Specifically, in plasma-assisted MBE smooth films are obtained under group-III rich conditions of growth. On the other hand in ammonia-MBE smooth films are obtained under nitrogen rich conditions of growth. High quality films were obtained on 6H-SiC without the employment of any buffer. The various nucleation steps used to improve the two dimensional growth as well as to control the film polarity were discussed. The n- and p-doping of GaN were addressed. The concept of increasing the solubility of Mg in GaN by simultaneously bombarding the surface of the growing film with a flux of electrons (co-doping GaN with Mg and electrons) was discussed. The influence of the strength of Al-N, Ga-N and In-N bonds on the kinetics of growth of nitride alloys was pointed out. Specifically, it was shown that in both the nitrogen-rich and group-III rich growth regimes, the incorporation probability of aluminum is unity for the investigated temperature range of 750-800° C. On the other hand the incorporation probability of gallium is constant but less than unity only in the nitrogen-rich regime of growth. In the group-III regime the incorporation probability of gallium decreases monotonically with the total group-III flux, due to the competition with aluminum for the available active nitrogen. Alloy phenomena such as phase separation and atomic ordering and the influence of these phenomena to the optical properties were addressed. InGaN alloys are thermodynamically unstable against phase separation. At compositions above 30% they tend to undergo partial phase separation. Furthermore, InGaN alloys were found to undergo 1x1 monolayer cation ordering. AlGaN alloys do not show evidence of phase separation but they were found to undergo multiple type of superlattice ordering. Under nitrogen-rich growth conditions they show one monolayer periodicity, while under group-III rich growth it was found that the structure is a superposition of a seven monolayer and twelve monolayer superlattices. Finally, the growth of heterostructures and MQWs and the use of the MBE method for the fabrication of optical, electronic and electromechanical devices were discussed.
Following topics are reviewed in this paper. After an introduction in section 1, section 2 reviews growth conditions of the most widely used III-nitride semiconductors, GaN and InGaN, by mean of MOCVD, and their optical properties are examined in conjunction with the carrier localization and the quantum confined Stark effects. A-face sapphire is now collecting more attention as a substrate for electronic devices, since it is available in very large size. The growth on A-face sapphire substrate is reviewed in section 3. Several MOCVD reactors with large capacity available on market are introduced in section 4. Both negative and positive aspects of the dislocation in GaN and InGaN are summarized in section 5. Although a dislocation works as a non-recombination center, it produces indium composition fluctuation of an InGaN and enhances carrier localization making light emission efficiency less sensitive to the presence of non-radiative recombination centers. Section 6 summarizes new technique to reduce dislocation density in GaN grown on heterogeneous substrates. And the paper is summarized in section 7.
Devices fabricated in the group-III nitride material system have shown significant resistance to the types of degradation common to those encountered in the GaAs and InP -based systems. As a result, GaN-based light emitting diodes have pushed the technological limits of package technology and design for high output power. The relatively high defect densities in these materials have been shown to be a device weakness for laser applications. This paper reviews these concepts to try to give the readers an understanding of the differences between the group-III nitrides and standard III-V materials.
In this paper we discuss the applicability of high-pressure grown bulk GaN crystals as substrates for device oriented MOVPE homoepitaxy. First, we fabricated light emitting diodes as a step towards realization of our target device: a blue light emitting laser diode. Our homoepitaxialy grown LEDs are characterized by excellent electrical characteristics and very satisfactory optical properties. Building on the experience gained during this first stage of our research we have been able to fabricate pulse current operated laser diodes emitting light at a wavelength between 397 and 430 nm. We believe that this fast progress clearly demonstrates the usefulness of bulk GaN substrates for optoelectronic devices, especially for high power laser diodes.
Numerous defects are generated in the heteroepitaxy of GaN, with threading dislocations (TDs) being the most prevalent. A novel method of reducing the defect density has been the Epitaxial Lateral Overgrowth (ELO) technology, where parts of the highly dislocated starting GaN is masked with a dielectric mask, after which growth is restarted. At the beginning of the second step, deposition only occurs within the openings with no deposition observed on the mask. This is referred as Selective Area Epitaxy (SAE). The TDs are prevented from propagating into the overlayer by the dielectric mask, whereas GaN grown above the opening (coherent growth) keeps the same TDs density as the template, for at least during the early stages of the growth.
Currently, two main ELO technologies exist: the simpler one involves a single growth step after stripe opening. In this one-step-ELO (1S-ELO), growth in the opening remains in registry with the GaN template underneath (coherent part), whereas GaN over the mask extends laterally (wings). This leads to two grades, namely highly dislocated GaN above the openings, and low dislocation density GaN over the masks. With this technique, devices have to be fabricated on the wings. Therefore, conversely, in the two-step-ELO (2S-EL0) process, the growth conditions of the first step are monitored to obtain triangular stripes. Inside these stripes, the threading dislocations arising from the templates are bent by 90° when they encounter the inclined lateral facet. In the second step, the growth conditions are modified to achieve full coalescence. In this two-step-ELO, only the coalescence boundaries are defective.
In depth characterisation of these ELO GaN layers reveals that the intermediate stages of the process induce an inhomogeneous impurity incorporation and stress distribution. However, the ELO technology produces high quality GaN, with TDs densities in the mid 106cn-2, linewidths of the low temperature photoluminescence (PL) near band gap recombination peaks below 1 meV, and deep electron traps concentration below 1014cm-3 (compared to mid 1015cm-3 in standard GaN). To further reduce the TDs density, multiple step ELO have also been implented.
For applications such as read/write laser light sources of digital versatile disks, higher power and longer operation lifetimes are required, thus necessitating the production of better quality material. Several options are also currently available to pave the way towards self supported high quality GaN. These technologies involve growing thick GaN layers (possibly on MOVPE ELO GaN) and then separating the layers from the substrate. HVPE has proven to be a reliable method to grow GaN with growth rates ranging from 30 to 100 pm/hour. In thick layers (several hundred μm), the mecanisms used for the reduction of dislocations become more efficient. Separation from the starting substrate is currently achieved by either laser lift off, chemically or by strain induced.
Recent advances in developing process modules for GaN photonic and power electronic devices are reviewed. These processes include damage removal in dry etched n- and p- GaN, implant doping and isolation, novel gate dielectrics, improved Schottky and ohmic contacts.
Understanding the mechanisms of optical gain in a semiconductor laser material is the key issue towards minimizing the threshold current density. For nitride-based laser structures, there has been a lively debate as to the role of fluctuations, polarization fields, and many-body effects in todays GalnN/GaN/AlGaN laser structures.
A thorough understanding of the fundamental materials properties forms the basis of any further consideration. We will then review the basic models, the theoretical approaches, and the available experimental evidence supporting the competing views of optical gain in the nitrides. The properties of guided optical modes in nitride waveguides will play an important role. A critical discussion of those results will finally allow us to discuss ultimate performance limits for laser diodes.
Wide-bandgap group-ill nitride lasers, which emit from near-ultraviolet to pure-blue, are reviewed. Characteristics of 405 nm wavelength laser diodes (LDs) are discussed. Reducing threading dislocation can increase the lifetime of nitride LDs. Using epitaxial lateral overgrowth technique, the dislocation density of the order of 105 cm-2 has been obtained. The relation between the lifetime of nitride LDs and the density of dislocation are discussed. Some optical and electrical properties are very important for optical disk system such as digital versatile disk (DVD) system. In use of the DVD system, important properties of nitride LDs are discussed. Furthermore, near-ultraviolet LDs and pure-blue LDs are described. The near-ultraviolet LDs uses GaN or AlInGaN active layer instead of In GaN.
The dilute nitrogen alloys GaAs1-xNx and GaP1-xNx have recently become technologically important for applications in high efficiency solar cells and vertical cavity surface emitting diode lasers used for fiber optic communications. There exist many inconsistencies between the results of various experimental techniques and theoretical models used to probe the giant band gap lowering observed in. these systems. It appears that these inconsistencies originate because GaAs1-xNx and GaP1-xNx should perhaps not be viewed as an abnormal alloys but rather as a heavy isoelectronically doped semiconductors. The similarity and dissimilarity between the two systems will be discussed with respect to: (1) The perturbation of the host band structure caused by nitrogen doping. (2) The evolution of nitrogen bound states with increasing nitrogen doping (3) The dominant contributors to the band edge absorption, and (4) Whether there exists a universal model that explains the anomalous behaviour of GaAs1-xNx and GaP1-XNX. Key issues such as the relevance of various theoretical band structure calculations to the experimentally measured parameters, and as to how exactly does one define the band gap for these materials will also be examined. Finally, possible solutions for regularizing the abnormal behavior of dilute N alloys will be discussed.
Nitride-based VCSELs are very attractive for extending the wavelength range of the parallel optoelectronic systems. The short-wavelength GaN-VCSELs are expected as lights, displays and parallel read/write heads of optical memories. On the other hand, the dilute-nitride GalnNAs on a GaAs will become a key for long-wavelength VCSELs utilized in high-speed and low cost network systems covering from very short to long-distance. Critical issues for realizing VCSELs are the formation of the high quality cavity as well as the growth of the high quality active layer. For the GaN-based system, the crystal quality of the active layer has been improved. Fabrication technologies of high reflectivity mirrors and a short cavity are necessary for the vertical cavity structure. For GalnNAs-based VCSELs, matured GaAs-based VCSEL technologies such as DBRs and selective oxidation are applicable. In these few years, the GalnNAs crystal quality have been improved and the VCSEL performances become practical level. In this paper, fabrication technologies of the vertical cavity for GaN-based VCSELs are presented and the current state of GalnNAs VCSELs is reviewed.
In the past few years, the wide-bandgap III-N compound semiconductor materials have been the subject of intense research owing to their commercial importance for the production of high-brightness light-emitting diodes. Another potentially important application for the Column III nitrides is for detection of ultra-violet radiation for various sensing, monitoring, and control applications. There has been a growing interest in back-illuminated solar-blind AlxGa1-xN photodiodes for flip-chip mounting to silicon read-out circuits. These devices not only need to have high external quantum efficiencies, but these efficiencies must be achieved at, or less than, the operating voltage of the readout display. This paper describes AlxGa1-xN heteroepitaxial back-illuminated p-i-n photodiodes that have been developed for these applications.