S. R. Forrest

Structural and morphological investigation of the development of electrical conductivity in ion‐irradiated thin films of an organic material

Thin films of 3,4,9,10‐perylenetetracarboxylic dianhydride (PTCDA) develop low electrical resistivity ( 5×1...

Relationship between the conduction‐band discontinuities and band‐gap differences of InGaAsP/InP heterojunctions

We have measured the magnitude of the conduction‐band discontinuities at heterojunctions for several compositions of InGaAsP grown lattice matched on InP. We find that the conduction‐band discontinuity (ΔEc) is related to the difference in band gaps (ΔEg) between the InGaAsP and InP layers via ΔEc =0.39(ΔEg). Thus, 40% of the band‐gap difference lies in the conduction band of this material system. The measurements were made on a series of composition of InGaAsP spanning the alloy range from In0.53Ga0.47As (with energy gap Eg =0.75 eV) to InP (Eg =1.35 eV) using capacitance‐voltage techniques. Depletion deep into the semiconductor layers was facilitated by the formation of organic‐on‐inorganic semiconductor contact barriers on the InGaAsP surface.

Performance of In x Ga 1-x As y P 1-y photodiodes with dark current limited by diffusion, generation recombination, and tunneling

We present a theoretical study of the effects of diffusion, generation-recombination (GR), and the recently observed tunneling currents on the performance of photodiodes made from In 0.73 Ga 0.27 As 0.63 P 0.37 and In 0.53 Ga 0.47 As. Calculations are made for both p+ν and p-i-n punch-through diode configurations, and are compared with recent measurements made by several independent investigators. For doping densities typical of present material ( N_{D} \gsim 10^{15} cm-3), tunneling currents become dominant prior to avalanche breakdown. Thus, for detection of weak (-55 dBm at 45 Mbits/s) optical signals, the diodes must be operated at low voltages where GR is the dominant source of reverse-biased leakage. To meet the requirements of low capacitance ( C \leq 0.5 pF for a diode area of 10-4cm2) and low GR dominated dark current ( I_{D} \leq 10 nA at T = 70\deg C), the doping density and effective carrier lifetime (τ eff ) must be N_{D} cm-3and \tau_{eff} \gsim 150 ns for In 0.73 Ga 0.27 As 0.63 P 0.37 and 5 \times 10^{14} \lsim N_{D} \lsim 7 \times 10^{15} cm-3and \tau_{eff} \gsim 3.5 \mu s for In 0.53 Ga 0.47 As.

Evidence for tunneling in reverse‐biased III‐V photodetector diodes

Photodiodes made from III‐V group semiconductor alloys have been found to exhibit anomalously high dark currents. We present evidence that tunneling is the dominant source of dark current in many cases. The tunneling current becomes substantial at peak junction electric fields as low as 105 V/cm due to the small direct energy gaps and small effective masses of the materials tested. Tunneling sets limits on the magnitude of the electric field attainable in these materials, and therefore has serious implications on photodetector design and performance.

Ion‐beam‐induced conductivity in polymer films

Polymer films darken upon irradiation with energetic ion beams. At high doses (1016–1017 cm−2) of 2‐MeV Ar+ ions, the resistivity of these insulating films decreases dramatically to 3.5×10−3 Ω cm. Furthermore, over a wide range of doses (1014–1015 cm−2) these films exhibit a temperature‐dependent resistivity characteristic of carrier transport via hopping between isolated conducting islands. Rutherford backscattering measurements indicate that while a substantial amount of the carbon is retained in the film, other constituent elements are lost. Raman spectra show that the films are highly disordered at large ion doses.

Optical and electrical properties of ion‐beam‐irradiated films of organic molecular solids and polymers

Thin films of several nonpolymeric and polymeric organic compounds become optically dark and electrically conducting on irradiation with energetic particle beams. Using 2‐MeV Ar+ ion beam irradiation, we have generated conducting patterns in otherwise high‐resistivity, organic thin films among which are 3, 4, 9, 10‐perylenetetracarboxylic dianhydride, 1, 4, 5, 8‐napthalenetetracarboxylic dianhydride, and Ni phthalocyanine. The room‐temperature resistivity of the films changes by 14 orders of magnitude from its as‐deposited value of ρ>1010 Ω cm to ρ=5×10−4 Ω cm at ion doses of 1017 cm−2. The temperature (T) dependence of the resistivity follows ρ(T)∝exp(+(T0/T)1/2 ) over a wide range of dose and temperature. The characteristic temperature T0 is found to be a function of dose. The observed behavior of ρ(T) is consistent with charge transport due to hopping between isolated, conducting islands. The dependence on ion energy, ion species, and ion dose rate has been examined. The effectiveness of an ionic speci...

Analysis of the dark current and photoresponse of In0.53Ga0.47As/InP avalanche photodiodes

The doping profiles, current-voltage (I–V) and photoresponse characteristics of five In0.53Ga0.47As/InP avalanche photodiode (APD) wafers are presented. A detailed analysis indicates that the dark current is due largely to generation and recombination of carriers in the diode bulk, and in some wafers tunneling at the p-n junction is dominant near breakdown (VB). In some cases, significant surface currents are also observed. In three high-performance wafers, however, low primary dark currents (∼5 nA) with no evidence for tunnelling at 0.99 VB have been obtained. In addition, microplasmas have been found in some wafers, due to local breakdown possibly arising from crystalline defects. Nevertheless, we report uniform gains as high as 100. The dark current and gain characteristics of these devices are among the best reported to date for In0.53Ga0.47As/InP APDs. Finally, the response of the APDs to fast optical pulses has been analyzed at both low and high illumination intensity. The slow speed of response, which has been reported elsewhere, is considered in detail and is found to be due to charge pile-up at the abrupt n-In0.53Ga0.47As/n-InP heterointerface which is characteristic of our devices. Using an analysis of the response time thermal activation energy along with the transient pulse shape, we infer that the heterointerfaces are graded over a length of 2L ≌ 300 A. The model predicts that fast response can be obtained for heterointerface grading lengths of 2L ⩾ 500 A, depending on the epitaxial layer doping and extent of penetration of the depletion region into the In0.53Ga0.47As layer at breakdown.

Monolithic optoelectronic integration: A new component technology for lightwave communications

We discuss recent advances in the field of optoelectronic device integration. Several problems and advantages associated with integration are illustrated by discussing in detail three device types which are currently undergoing intensive investigation: integrated laser transmitters, integrated p-i-n photodetector receivers, and arrays of individually addressable detectors and light emitters. Devices fabricated using either GaAs or InP-based material systems with application at wavelength of0.82-0.87 \mum and1.3-1.55 \mum, respectively, are considered. It is concluded that the pursuit of optoelectronic integration will lead to an increase in device functionality, an improvement in performance, and a reduction in cost of the integrated device as compared with its hybrid counterpart.

Semiconductor analysis using organic‐on‐inorganic contact barriers. II. Application to InP‐based compound semiconductors

Organic‐on‐inorganic (OI) contact barrier devices have been applied to the study of InP and In0.53Ga0.47As surfaces. The characteristics of these devices differ from OI diodes fabricated using Si or Ge substrates in that the contact barriers for InP‐based devices are relatively small (≤0.55 eV), and the diode characteristics are governed by a high density of states at the organic/inorganic interface. We present current‐voltage and frequency‐dependent admittance‐voltage characteristics for OI diodes employing 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA) and related compounds as the organic thin‐film material. Analysis of characteristics using a theory presented previously [S. R. Forrest and P. H. Schmidt, J. Appl. Phys. 59, 513 (1986)] indicates that the surface state density is (i) independent of the organic material employed, and (ii) sensitive to the exposure of the surface to chemical treatment prior to the organic thin‐film deposition. Using techniques derived previously, we determine the magn...

Performance of In 0.53 Ga 0.47 As/InP avalanche photodiodes

We calculate operating characteristics of high-sensitivity high-speed In 0.53 Ga 0.47 As/InP avalanche photodiodes (APDs). We find that significant photocurrent gain is obtained for a total fixed-charge density of \sigma_{tot} > 3 \times 10^{12} cm-2in the depleted InP and 0.53 Ga 0.47 As regions. To obtain high quantum efficiency and low tunneling currents, the fixed-charge density in the InP must be in the range 2 \times 10^{12} cm-2 \leq \sigma_{B} \leq 3 \times 10^{12} cm-2. We calculate the breakdown voltages for APDs with uniformly doped layers and find that practical detectors with avalanche breakdowns as low as 15 V can be realized. High quantum efficiency and fast response are obtained by compositional grading of the In 0.53 Ga 0.47 As heterointerface over a distance of L \gsim 380 A, depending on the doping and amount of the In 0.53 Ga 0.47 As layer swept out at breakdown. Finally, a comparison of calculations with experimental results is presented.