1740 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 34, NO. 9, SEPTEMBER 1998 Material and Ultrafast Optoelectronic Properties of Furnace-Annealed Arsenic-Ion-Implanted GaAs Gong-Ru Lin, Member, IEEE, Wen-Chung Chen, C.-S. Chang, Shyh-Chin Chao, Kaung-Hsiung Wu, T. M. Hsu, W. C. Lee, and Ci-Ling Pan, Member, IEEE Abstract—Structural, electrical, and ultrafast optical properties of furnace-annealed arsenic-ion-implanted GaAs (GaAs : As ) has been investigated for its applications in ultrafast optoelectron- ics. From these studies, we determine that GaAs substrates im- planted with 200-keV arsenic ions at 10 ions/cm and furnace- annealed at 500 C–600 C would have recovered its crystallinity, be highly resistive, and exhibit picosecond photo-excited carrier lifetimes. The duration of the electrical pulses generated by photoconductive switches (PCS’s) fabricated on the optimized material was 4 ps. The risetime (10%–90%) and 1/e falltime were, respectively, 2 and 3 ps. These results were measurement- system limited. We estimated the actual response to be 2 ps, consistent with a photo-excited carrier lifetime of 1.8 ps. The peak responsivity was 4 10 A/W. The dark current for the GaAs : As PCS biased at 40 V was as low as 5 nA. The break down field was higher than 150 kV/cm. These characteristics are comparable to those of state-of-the-art photoconductors such as LT-GaAs. Index Terms— Arsenic-ion-implanted GaAs, photoconductive switch, ultrafast optoelectronics. I. INTRODUCTION P HOTOCONDUCTORS with ultrashort photo-excited car- rier lifetime, good optical responsivity, high breakdown field, and low dark current are essential for ultrafast optoelec- tronic switching applications. Various classes of semiconduc- tors, e.g., intrinsic, impurity-dominated, radiation-damaged, polycrystalline, and amorphous, have been explored as ul- trafast photoconductors [1]. In particular, the ion implanta- tion technique has been employed extensively [2]–[5]. Car- rier lifetimes as short as 0.5–0.6 ps have been reported as the saturation limit for GaAs samples irradiated by pro- tons (H ) [2]. Similar results were reported for oxygen-ion- implanted silicon-on-sapphire (SOS) materials [5]. On the other hand, nonstoichiometric arsenic-rich GaAs grown by molecular beam epitaxy (MBE) at low substrate tempera- Manuscript received September 8, 1997; revised April 8, 1998. This work was supported in part by the National Science Council (NSC) of the Republic of China under Grant NSC84-0212-M009-002 and Grant 84-2215-E-009-092. G.-R. Lin is with the Institute of Electro-Optical Engineering, Tatung Institute of Technology, Taipei 104, Taiwan, ROC. W.-C. Chen, C.-S. Chang, and C.-L. Pan are with the Institute of Electro- Optic Engineering, National Chiao Tung University, Hsinchu, Taiwan 30010, ROC. S.-C. Chao and K. H. Wu are with the Department of Electro Physics, National Chiao Tung University, Hsinchu, Taiwan 30010, ROC. T. M. Hsu and W. C. Lee are with the Department of Physics, National Central University, Chung-Li, 32054 Taiwan, R.O.C. Publisher Item Identifier S 0018-9197(98)06386-6. ture (LT-GaAs) have been the subject of intensive studies [6]. It exhibited nearly ideal electrical and optoelectronic properties for ultrafast optoelectronic applications. Recently, a new class of arsenic-rich material, arsenic-ion-implanted GaAs or GaAs : As , has emerged as a potential alternative to LT-GaAs [7]. The structural and electrical characteristics of LT-GaAs and GaAs : As have been shown to be quite similar [8], [9]. The ultrafast optical characteristics of the as-implanted arsenic-rich GaAs materials have been reported [10]–[12]. Subpicosecond carrier lifetimes were observed. However, as-implanted or as-grown nonstoichiometric GaAs obtained by using either the arsenic-ion-implantation or the LTMBE process suffer from the following: the crystallinity of the material is degraded [13], [14]. As a result, the carrier mobility is substantially lower [12], [15]. This also leads to a defect-dominated hopping conducting mechanism, which destroys the highly resistive nature of the original substrate [8], [15]. Consequently, the as-implanted GaAs : As is not yet suitable for fabrication of ultrafast optoelectronic devices. To remedy this, rapid thermal annealing (RTA) has been success- fully employed. The effects of RTA on photo-excited carrier lifetime and electrical properties of GaAs : As were reported [16]. Ultrafast photoconductive switches were demonstrated on RTA-annealed GaAs : As prepared by bombarding semi- insulating (SI) GaAs substrates with 200-keV arsenic ions at the dosage of 10 ions/cm [10], [16]. We have also reported picosecond photoconductive switches fabricated on low-dose- implanted GaAs : As [17]. An interesting alternative to RTA is furnace annealing. In this paper, we present a detailed study on the structural, electrical, and ultrafast optical properties of furnace-annealed GaAs : As . The optimum processing condition is determined to be furnace-annealing at 600 C for 30 min. Picosecond photoconductive switches (PCS’s) were fabricated on this material. The devices exhibit characteristics better than or comparable to those of LT-GaAs. II. SAMPLE PREPARATION The substrates were liquid-encapsulated-Czochralski (LEC) grown SI GaAs wafers. For implantation of arsenic ions, we employ a commercial apparatus (Varian E220). The implant- ing dosage and energy were 10 ions/cm and 200 keV, respectively. Typical current density value was 1 mA/cm . The exposure time subject to the samples was 10–15 min. The postannealing process was performed ex situ by the encap- sulated thermal annealing technique. We used a quartz-tube 0018–9197/98$10.00  1998 IEEE