Barrier Photodetectors for High Sensitivity and High Operating Temperature Infrared Sensors Philip Klipstein
General Review of Barrier Detectors 1) Higher operating temperature, T OP 2) Higher signal to noise at low T OP 3) Simple 2-color architecture 4) Simplification of processing and passivation Standard Diode Barrier diode SPIE 6940-92 19 March 2008 2
Timeline 1983: Anthony White (UK): patent of barrier devices in MCT: US 4,679,063 2003: P C Klipstein, patent of barrier devices in InAsSb and superlattice: WO 2005/004243 A1 2006: Maimon and Wicks, Appl. Phys. Lett. 89, 151109 Coined the name: nbn First nbn fabrication in InAs 2007: U of New Mexico, Appl. Phys. Lett91, 043514 First nbn fabrication in Superlattice SPIE 6940-92 19 March 2008 3
BIPOLAR X Bn Detector Family - No Depletion in NARROW bandgap absorber - Holes free to move from absorber to contact p-b-n UNIPOLAR n-b-n BIPOLAR UNIPOLAR C p -B-n C n -B-n SPIE 6940-92 19 March 2008 4
Standard Diode: NARROW bandgap is depleted GR process S-R-H trap (G-R centre) Diffusion process E G IDiffusion IGR ~ ~ EG kbt e EG 2kBT e I = I + I Diffusion GR 77K L n L dep L p Log I E /k /kslopes G B slopes I MWIR (λ C ~4µ) at 77K: 1 N L τ E /2k GR D Dep p0 4 5 ~ ~ 10 10 IDiffusion 2 ni L τ p n0 SPIE 6940-92 1 / T 19 March 2008 5 G B
XBn has no depletion in narrow bandgap S/C XBn has negligible G-R current Log I I STD XBn (high temperature) Dark Current Arrhenius plot Standard diode XBn (high sensitivity) XBn Device only has an advantage below T 0 T 0 1 / T I STD < I photo SPIE 6940-92 19 March 2008 6
E C E (p) F E V GaSb + ++ + ++++ ----- AlSbAs V bias MWIR CBn detector Room Temperature Bandgap (ev) 2.4 2.0 1.6 1.2 0.8 0.4 GaP Si AlP GaAs AlAs Ge InP AlSb GaSb InAs Indirect Gap Direct Gap 0.517 0.563 0.620 0.689 0.775 0.885 1.033 1.240 1.550 2.067 3.100.. InSb 6.200 0 0.54 0.55 0.56 0.57 0.58 0.59 0.60 0.61 0.62 0.63 0.64 0.65 Lattice Constant (nm) Room Temperature Wavelength (microns) InAsSb E C E F(n) SPIE 6940-92 19 March 2008 7 E V i) Optical behaviour like narrow gap diode ii) Electrical behaviour like wide gap diode
log 10 I J -4-6 -8-10 -12 GR Estimation of crossover temperature, T 0 Ldep NvNc = 2 τ τ InSb p 0 n 0 2 4 6 8 10 12 14 1000/T ( K -1 ) e T0(K) 250 200 150 100 2k T 50 E G B J GR (T 0 )= J diff (T 0 ) τ n0 ~300nS D P ~32cm 2 /s (τ p0 ~500nS) T 0 ~130K N=2e16 N=2e15 0.2 0.3 0.4 Bandgap (ev) SPIE 6940-92 19 March 2008 8 J Diff ~ InAs 0.91 Sb 0.09 T 0 ~170K NC N N D V L τ diff p p 0 e E B G k T V.P. Astakhov et al. Pis ma Zh Tekh Fiz 18, 1 (1992) InAs
Estimation of XBn Operating Temperature, T 0P Temperature (K) 180 170 160 150 140 130 120 λ C = 4.2µ (E G =0.3eV) T 0 1.E+14 1.E+15 1.E+16 1.E+17 N D (cm -3 ) T OP: BLIP/10 (f/3;qe=0.7) InAs 0.91 Sb 0.09 BLIP/10: J diff (T OP )= J photo /10 T OP < T 0, so barrier detector works HOTTER than standard detector Real T OP ~ 150K is feasible for large N D Barrier detector tolerates large N D because depletion width is independent of N D (determined by barrier width) and bias SPIE 6940-92 19 March 2008 9
C p -B-n AlSb As 1-x x Bias Range E C p-type GaSb V bias =V MAX E C V bias <V MAX E (p) F E V ξ V bias InAs Sb 1-z z E (n) F E C E V E F(p) E V < 3kT OP V bias E (n) F E C E V No G-R current = higher T OP SPIE 6940-92 19 March 2008 10
C p -B-n Barrier Doping E C barrier p-type GaSb AlSb As 1-x x Flat bands, in InAsSb p-type barrier Bent bands, in InAsSb E (p) F E V ξ V bias InAs Sb 1-z z E (n) F E C E V Hole accumulation layer V BIAS Depletion in absorbing layer = lower T OP SPIE 6940-92 19 March 2008 11
Type II SL CBn detectors Al Ga Sb As 1-x x 1-y y SL( ) λ 1 SL( ) λ 2 E F,L V bias E F (n) λ 1 λ 2 E F,R C p -B-n C n -B-n (2 colour) SPIE 6940-92 19 March 2008 12
Type II SL CBn detectors Al Ga Sb As 1-x x 1-y y SL( ) λ 1 SL( ) λ 2 E F,R E F,L V bias E F (n) λ 1 λ 2 C p -B-n C n -B-n (2 colour) SPIE 6940-92 19 March 2008 13
Processing of XBn Detector B C p n-active layer Etch stop B C p n-active layer Cap B Metal C p n-active layer I Metal -Barrier acts a bit like overgrown passivation layer -Standard LWIR FPAs are hard to passivate XBn could be a good alternative passivation solution SPIE 6940-92 19 March 2008 14
C p -B-n Limits on Barrier width and height t. emission GaSb E C ~1.0eV n-b-n t. emission E C ~2.1eV η V bias tunneling InAs Sb 0.91 0.09 InAsSb tunneling InAs Sb 0.91 0.09 InAsSb I TE, I tunn < I photo (f/8, QE=0.7) gives Thermionic emission: E C >0.4eV (based on Richardson Formula) Tunneling: Barrier Thickness > 300Å (based on 2-band k.p calculation) SPIE 6940-92 19 March 2008 15
Conclusions 1. XBn Detectors come as CBn, pbn, nbn, etc. 2. No depletion in absorbing layer and no G-R current 3. Anticipated Advantages for MW and LW i) HOTTER than a standard photodiode (MWIR) ii) LESS NOISE at lower T OP (MWIR) iii) EASIER to passivate (LWIR) iv) 2 color (including MW/LW) is simple in nbn configuration v) All proposed designs grown on GaSb substrates 4. XBn detector design for better performance : barrier and high doping in absorbing layer SPIE 6940-92 19 March 2008 16