Recent advances in nonsilica fiber technology have prompted the development of suitable materials for devices operating beyond 155 mu m The III-V ternaries and quaternaries (AlGaIn)(AsSb) lattice matched to GaSb seem to be the obvious choice and have turned out to be promising candidates for high speed electronic and long wavelength photonic devices Consequently, there has been tremendous upthrust in research activities of GaSb-based systems As a matter of fact, this compound has proved to be an interesting material for both basic and applied research At present, GaSb technology is in its infancy and considerable research has to be carried out before it can be employed for large scale device fabrication This article presents an up to date comprehensive account of research carried out hitherto It explores in detail the material aspects of GaSb starting from crystal growth in bulk and epitaxial form, post growth material processing to device feasibility An overview of the lattice, electronic, transport, optical and device related properties is presented Some of the current areas of research and development have been critically reviewed and their significance for both understanding the basic physics as well as for device applications are addressed These include the role of defects and impurities on the structural, optical and electrical properties of the material, various techniques employed for surface and bulk defect passivation and their effect on the device characteristics, development of novel device structures, etc Several avenues where further work is required in order to upgrade this III-V compound for optoelectronic devices are listed It is concluded that the present day knowledge in this material system is sufficient to understand the basic properties and what should be more vigorously pursued is their implementation for device fabrication (C) 1997 American Institute of Physics

The physics and technology of gallium antimonide: An emerging optoelectronic material

The first superlattice avalanche photodiode (APD) is reported. The high field region of this p‐i‐n structure consists of 50 alternating Al0.45Ga0.55As (550 A) and GaAs (450 A) layers. A large ionization rate ratio has been measured in the field range (2.1–2.7)×105 V/cm, with α/β≃10 at a gain of 10 giving a McIntyre noise factor Fn = 3. The ionization rate ratio enhancement with respect to bulk GaAs and AlGaAs is attributed to the large difference in the band edge discontinuities for electrons and holes at the heterojunction interfaces. The superlattice APD is a new device concept which can be used to develop low noise APD’s in a variety of III‐V materials including long wavelength 1.3–1.6‐μm semiconductors.

Enhancement of electron impact ionization in a superlattice: A new avalanche photodiode with a large ionization rate ratio

The theory of the staircase avalanche photodiode (APD) is presented and recent results on a new class of APD's with enhanced ratio of ionization coefficients are reviewed. The staircase APD consists of a multistage graded gap structure where only electrons ionize; the entire ionization energy is provided by large conduction band steps (dynodes). A general expression for the excess noise factor F in terms of the number of stages and the multiplication per stage is presented. For high ionization yields per dynode the F factor is near unity independently of the number of stages, implying virtually noise free multiplication at high gain similar to a photomultiplier. This cannot be achieved in a conventional APD at high gain even if one of the ionization coefficients is zero. A comparison between the noise behavior of the staircase APD and that of a phototube is also presented. A microscopic theory of the ionization yield γ is discussed; to obtain a high γ electrons must approach the dynode with an energy in the order of ten times the optical phonon energy. The possible problem of residual hole-initiated ionization is also discussed. Formulas for the electron and hole initiated multiplications are derived; from a measurement of these quantities one can directly obtain the ionization yield and the residual hole ionization coefficient. Experimental and theoretical results on other structures (superlattice, channeling, graded gap APD's) with high \alpha/\beta ratio are also reviewed and design considerations for a long-wavelength multilayer APD are presented.

Staircase solid-state photomultipliers and avalanche photodiodes with enhanced ionization rates ratio

Receiver design for high-speed ptical-fiber systems

The technology of optical-fiber systems is advancing rapidly. Parallel to the development of long-haul telecommunication systems in the gigabits per second data rates operating in the long-wavelength region is the wide penetration of optical-fiber systems in local area networks, video trunking and distribution, sensors, etc. These diversified applications impose different and often conflicting constraints on the optical receiver. This paper re-examines the optical receiver design in view of these different requirements, namely, high receiver sensitivity, wide dynamic range, transparent to the operating bit rate, unrestricted data format, and fast acquisition time. Design tradeoffs between conflicting receiver requirements are considered in detail. In particular, the sensitivity of high-capacity long-wavelength transmission systems is emphasized. The state-of-the-art performance of photodetectors and low-noise amplifiers is discussed. We show that dark current of avalanche photodiodes (APD's) is the main factor limiting the sensitivity of long-wavelength optical receivers. In addition, as an example, we report on the design and experimental performance of a hybridized low-noise optical receiver amplifier capable of more than 2-Gbits/s operation. The input noise spectral density achieved is 9 pA/ \sqrt{Hz} with a noise corner frequency of 920 MHz, corresponding to an equivalent noise resistance of 120 Ω.

Receiver design for high-speed optical-fiber systems

The liquid-phase epitaxy and device fabrication of p-n and p-i-n Ga 1-x Al x Sb avalanche photodiodes is described. Breakdown voltages up to 95 V and dark currents of 10-4A/cm2have been obtained. With p-i-n diodes we have measured the impact ionization coefficients α (electrons) and β (holes) with different composition and temperature. A resonant enhancement of the hole ionization coefficient is found for x = 0.065 (300 K) where the ratio \beta/\alpha exceeds values of 20. This effect is attributed to impact ionization initiated by holes from the split-off valence band: if the spin orbit splitting Δ is equal to the bandgap energy E g , the threshold energy for hole initiated impact ionization reaches the smallest possible value ( E_{i} = E_{g} ) and the ionization process occurs with zero momentum. This leads to a strong increase of β at \Delta/E_{g} = 1 . The experimentally determined dependence of ionization coefficients on threshold energy is compared with theoretical expectations.

Ga 1-x Al x Sb avalanche photodiodes: Resonant impact ionization with very high ratio of ionization coefficients

For both sizes of diodes, on-the-wafer yields of APD’s with gain have been as high as 75 percent, which indicates a high level of bulk uniformity. Subnanosecond rise and fall times, and quantum efficiencies of 70 percent at 1.27 pm have also been obtained. The leakage current  density  at half the breakdown voltage is extremely low, even for the larger area diodes, typical values being in  the range 2-6 X A/cm2. However, at about  75  percent of the  breakdown voltage, the leakage current begins to increase rapidly, and at high gains leads to excessive shot noise. The  optimum  operating  point when used with a good low noise, broad-band (100 MHz), transimpedance amplifier typically occurs at a current gain of only 3-4, where the noise equivalent power of the APD is about 2 X lo-’’ W/Hzl/’. Noise measurements have been made and analyzed in order to determine whether the increase in leakage current is due to locally large multiplication of dark current at microplasmas, or whether it is simply due to an increase in the premultiplication dark current followed by uniform multiplication at the bulk  gain. The results indicate  that he increased  leakage current arises from a uniformly  multiplied,  but increasing value of the primary (i.e., premultiplication) leakage current. The size of the primary leakage current can be derived from the noise measurements, and leads to a prediction of gain saturation in approximate agreement with the observed maximum gain. There are some indications that the leakage current is surface dominated. Guard-ring structures are being fabricated in an effort to reduce surface effects. Preliminary structures with a surface breakdown voltage 35 V higher than the interior breakdown voltage (z 5 0 V) have been fabricated, and have resulted in gains up to about 10.

K.W. Benz

Papers

Ga 1-x Al x Sb avalanche photodiodes: Resonant impact ionization with very high ratio of ionization coefficients

TP-C13 ionization coefficients for electrons and holes in GaSb p-i-n and InP Schottky avalanche photodiodes