Surface voltage and surface photovoltage - Dieter Schroder ...

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Surface voltage and surface photovoltage in GaAs. In amorphous Si L n is even shorter. In such a situation the intercept is given by [28] ( 1 α =−L (W/L n ) 2 ) n 1+ . (24) 2(1+W/L n ) For W ≫ L n the 1/α intercept is −W/2, independent of the diffusion length. For W = L n the intercept becomes −5L n /4. The scr width can be reduced by shining steady-state light onto the device. What role does surface recombination play in diffusion length measurements? To answer this question, we have to look at the dependence of the recombination lifetime on bulk and surface recombination. The effective recombination lifetime is [29] 1 = 1 + 1 (25) τ eff τ B τ S with the bulk lifetime τ B and the surface lifetime τ S given by τ B = τ p(n 0 + n 1 + n) + τ n (p 0 + p 1 + p) p 0 + n 0 + n τ S = d 2s r (26) where n 0 and p 0 are the equilibrium carrier densities, p the excess carrier density, d the sample thickness, s r the surface recombination velocity and τ p = 1 σ p ν th N T τ n = 1 σ n ν th N T n 1 = n i e (ET −Ei)/kT p 1 = n i e −(ET −Ei)/kT (27) with σ p and σ n the hole and electron capture cross sections, v th the thermal velocity, N T the impurity density and E T the energy level of the impurity. The τ S term in equation (26) is valid for s r
D K Schroder (a) L n (µm) L n (µm) 5.2. Lifetime Two types of lifetime are measured in Si wafers: the recombination lifetime, τ r , and the generation lifetime, τ g [29]. As the name implies, the recombination lifetime characterizes the recombination of ehps, while the generation lifetime characterizes the generation of ehps. The recombination lifetime is determined by depositing sufficient charge on an oxidized wafer to invert the semiconductor surface forming a field-induced np junction in a p-type substrate. A brief light pulse forward biases this np junction, injecting excess carriers that subsequently recombine. The resulting time-dependent surface voltage is monitored with a Kelvin probe. This method is very similar to the open-circuit voltage decay technique. τ r is determined from the equation [37] τ r = kT/q dV p /dt . (30) (b) N Fe (x10 11 cm -3 ) (c) Figure 15. Diffusion length maps (a) before, (b) after Fe–B pair dissociation; (c) iron density map (in units of 10 11 cm −3 ). Courtesy of P Edelman and J Lagowski, Semiconductor Diagnostics, Inc. those of Fe–B. By measuring the diffusion length before (L ni ) and after (L nf ) Fe–B pair dissociation, the iron density N Fe is obtained from the expression [34] N Fe = 1.05 × 10 16 ( 1 L 2 nf − 1 ) L 2 cm −3 (29) ni with diffusion lengths in units of µm. The Fe i and Fe–B diffusion lengths as a function of Fe density are shown in figure 14 [35]. This large difference in diffusion length between Fe i and Fe–B only pertains at low injection levels and disappears at medium to high injection levels. Maps of diffusion lengths before and after Fe–B pair dissociation and the resultant Fe density are shown in figure 15. Metals rarely pair with impurities in n-type Si, although Cu does form such pairs [36]. The generation lifetime is determined by depositing a charge pulse driving the corona–oxide–semiconductor (COS) device into deep depletion. Electron–hole pairs are subsequently generated and the resulting voltage transient is monitored with a contactless Kelvin probe [23]. These two methods were recently used to characterize both epitaxial films and their substrates [38]. The epitaxial layer is characterized through τ g measurements, with the thermal carrier generation confined to the charge-induced space-charge region, which is typically of the order of 1 µm below the semiconductor surface. The recombination lifetime, on the other hand, characterizes a depth determined by the minority carrier diffusion length. Figure 16 illustrates corona-induced generation and recombination lifetime measurements of n-epitaxial layers on n-substrates. Figure 16(a) shows the results for both ‘good’ layer and ‘good’ substrate with τ g ≈ 4 ms and τ r ≈ 300 µs. Figure 16(b) shows a ‘good’ epitaxial layer (τ g ≈ 3ms), but the substrate is contaminated, reducing the recombination lifetime to about 50 µs. While recombination lifetime measurements can be made without corona charge, using conventional surface photovoltage or photoconductance decay measurements, contactless generation lifetime measurements require corona charge. 5.3. Charge The surface voltage dependence on surface charge lends itself to measurements of charge in the insulator on a semiconductor wafer or charge on the wafer. This charge can be oxide charge, interface trapped charge plasma damage charge, or other charge. Let us illustrate this by considering the mobile charge density Q m in an oxidized wafer [39]. Such mobile charge in silicon technology is most commonly sodium, originating from such sources as humans, chemicals, equipment etc. Other possible mobile charges are potassium, associated with chemical polishing compounds, and lithium, which can originate from pump oils. One way to measure such a mobile charge is to combine SV measurements with corona charge techniques by depositing corona charge on an oxidized semiconductor surface. First deposit positive corona charge, then heat the wafer to a moderate temperature of around 200 ◦ Cforafew R24
Page 1 and 2: INSTITUTE OF PHYSICS PUBLISHING MEA
Page 3 and 4: D K Schroder Vibrating Ref. Electro
Page 5 and 6: D K Schroder Figure 5. (a) Band dia
Page 7: D K Schroder As is obvious from the
Page 11 and 12: D K Schroder φ s (V) 1 0.5 0 -0.5
Page 13 and 14: D K Schroder Φ (photons/sÂP 2 ) 4
Page 15 and 16: D K Schroder Appendix B. Sinusoidal

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